Broadband or mid-infrared fiber light sources

ABSTRACT

An optical system for use in an imaging procedure includes one or more semiconductor diodes configured to generate an input signal beam with a wavelength shorter than 2.5 microns that is amplified and communicated through optical fiber(s) to a nonlinear element configured to broaden the spectral width to at least 50 nm through a nonlinear effect. A subsystem includes lenses or mirrors to deliver an output beam having a broadened spectrum to an Optical Coherence Tomography apparatus with a sample and reference arm to perform imaging for characterizing the sample. The delivered output beam has a temporal duration greater than about 30 picoseconds, a repetition rate between continuous wave and Megahertz or higher, and a time averaged intensity of less than approximately 50 MW/cm 2 . The output beam has a time averaged output power of 20 mW or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/750,556 filed Jan. 25, 2013 (now U.S. Pat. No. 8,670,642), which is acontinuation of U.S. application Ser. No. 13/241,900 filed Sep. 23, 2011(now U.S. Pat. No. 8,391,660), which is a Continuation of U.S.application Ser. No. 12/366,323 filed Feb. 5, 2009 (now U.S. Pat. No.8,055,108), which is a Continuation of U.S. application Ser. No.11/599,950 filed Nov. 15, 2006 (now U.S. Pat. No. 7,519,253), whichclaims priority to U.S. Provisional Patent Application Ser. No.60/738,389, filed Nov. 18, 2005, the disclosures of which are herebyincorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.W31P4Q-05-C-0159 awarded by the U.S. Army Aviation and Missile Commandand Contract No. W911NF-04-C-0078 awarded by the U.S. Army Robert MorrisACQ CTR. The Government has certain rights in the invention.

TECHNICAL FIELD

This invention relates in general to broadband or mid-infrared lightbased systems, and more particularly to a system and method forgenerating wavelengths between approximately 0.4 to 5 microns or morebased on fiber optic technologies or optical waveguides.

BACKGROUND AND SUMMARY

Broadband light sources, super-continuum sources, and Mid-Infrared FiberLight (MIRFIL) sources are described that generate wavelength in themid-infrared (mid-IR being wavelengths substantially between 2 to 5microns) based on nonlinear processes in optical fibers. Examples ofnonlinear processes in optical fibers include super-continuum (SC)generation, modulational instability (MI), cascaded Raman wavelengthshifting (CRWS), and four-wave mixing (4WM). Examples of optical fibersinclude fused silica fibers, fluoride fibers, chalcogenide fibers, andtellurite fibers.

Current techniques of generating mid-IR light include the use of opticalparametric oscillators (OPOs) or optical parametric amplifiers (OPAs).However, OPOs and OPAs are generally expensive, complicated, and involvemoving parts that are prone to mis-alignment. Alternative techniques forgenerating mid-IR light involve the use of quantum cascade lasers (QCL).However, QCL's are generally difficult to operate at wavelengths shorterthan about 4.4 microns, they put out low output powers, they haverelatively low efficiency, and they often required pulsed operation orcryogenic cooling.

A simpler technique for generating mid-IR light is to use laser diodesto pump optical fibers. The MIRFIL can exemplary involve the generationof mid-IR light in optical fibers by pumping with a variety of lasersincluding laser diodes, solid state lasers, or cladding-pumped fiberlasers. In one embodiment, SC generation is achieved to simultaneouslygenerate a wide band of wavelengths, which can advantageously be used tomimic the black body radiation of hot metal objects or to performspectral fingerprinting to identify one or more chemical species. Thefiber based MIRFIL can be lighter, more robust, more compact, simplerand less costly than the OPA or OPO alternatives. Moreover, the MIRFILcan produce a single spatial mode with minimal requirements for opticalalignments. In a preferred embodiment, nanosecond pulses are used togenerate mid-IR light. In addition, the MIRFIL approach leverages theenormous investment in telecommunications technologies and the maturefiber platform.

In one embodiment, an optical system for use in an imaging procedureincludes one or more semiconductor diodes configured to generate aninput beam, wherein at least a portion of the input beam comprises awavelength shorter than about 2.5 microns, one or more opticalamplifiers configured to receive at least the portion of the input beamand to communicate an intermediate beam to an output end of the one ormore optical amplifiers, and one or more optical fibers configured toreceive at least a portion of the intermediate beam and to communicateat least the portion of the intermediate beam to a distal end of the oneor more optical fibers to form a first optical beam. A nonlinear elementis configured to receive at least a portion of the first optical beamand to broaden a spectrum associated with the at least a portion of thefirst optical beam to at least about 50 nm through a nonlinear effect inthe nonlinear element to form an output beam with an output beambroadened spectrum. A subsystem includes one or more lenses or mirrorsconfigured to receive a received portion of the output beam and todeliver a delivered portion of the output beam to a sample to performimaging for characterizing the sample. The subsystem includes an OpticalCoherence Tomography (OCT) apparatus comprising a sample arm and areference arm, wherein the delivered portion of the output beam has atemporal duration greater than approximately 30 picoseconds, arepetition rate between continuous wave and Megahertz or higher, and atime averaged intensity of less than approximately 50 MW/cm². The outputbeam has a time averaged output power of about 20 mW or more.

In yet another embodiment, an optical system for use in an imagingprocedure having a plurality of semiconductor diodes, each of the diodesconfigured to generate an optical beam, a beam combiner configured toreceive at least a portion of the optical beams from the plurality ofsemiconductor diodes and to generate a multiplexed optical beam, anoptical fiber configured to receive at least a portion of themultiplexed optical beam and to communicate the at least a portion ofthe multiplexed optical beam to form an intermediate beam having atleast one wavelength, and a light guide configured to receive at least aportion of the intermediate beam and to propagate the at least a portionof the intermediate beam to form an output beam. A subsystem includesone or more lenses or mirrors configured to receive a received portionof the output beam and to deliver a delivered portion of the output beamto a sample to perform imaging for characterizing the sample, whereinthe subsystem comprises an Optical Coherence Tomography (OCT) apparatuscomprising a sample arm and a reference arm. The delivered portion ofthe output beam has a temporal duration greater than approximately 30picoseconds, a repetition rate from continuous wave to Megahertz orhigher, and a time averaged intensity of less than approximately 50MW/cm². The output beam has a time averaged output power of about 20 mWor more.

Another embodiment includes an optical system for use in a spectroscopyprocedure having one or more semiconductor diodes configured to generatean input beam, wherein at least a portion of the input beam comprises awavelength shorter than about 2.5 microns, one or more opticalamplifiers configured to receive at least the portion of the input beamand to communicate an intermediate beam to an output end of the one ormore optical amplifiers, and one or more optical fibers configured toreceive at least a portion of the intermediate beam and to communicateat least the portion of the intermediate beam to a distal end of the oneor more optical fibers to form a first optical beam. A nonlinear elementis configured to receive at least a portion of the first optical beamand to broaden a spectrum associated with the at least a portion of thefirst optical beam to at least about 50 nm through a nonlinear effect inthe nonlinear element to form an output beam with an output beambroadened spectrum. A subsystem includes one or more lenses or mirrorsconfigured to receive a received portion of the output beam and todeliver a delivered portion of the output beam to a sample to performspectroscopy for characterizing the sample based on chemicalcomposition, wherein at least a part of the sample comprises skin ortissue. The delivered portion of the output beam has a temporal durationgreater than approximately 30 picoseconds, a repetition rate betweencontinuous wave and Megahertz or higher, and a time averaged intensityof less than approximately 50 MW/cm². The output beam has a timeaveraged output power of about 20 mW or more.

One embodiment includes an optical system for use in a spectroscopyprocedure having a plurality of semiconductor diodes, each of the diodesconfigured to generate an optical beam, a beam combiner configured toreceive at least a portion of the optical beams from the plurality ofsemiconductor diodes and to generate a multiplexed optical beam, anoptical fiber configured to receive at least a portion of themultiplexed optical beam and to communicate the at least a portion ofthe multiplexed optical beam to form an intermediate beam having atleast one wavelength, and a light guide configured to receive at least aportion of the intermediate beam and to propagate the at least a portionof the intermediate beam to form an output beam. A subsystem includesone or more lenses or mirrors configured to receive a received portionof the output beam and to deliver a delivered portion of the output beamto a sample to perform spectroscopy for characterizing the sample basedon chemical composition, wherein at least a part of the sample comprisesskin or tissue. The delivered portion of the output beam has a temporalduration greater than approximately 30 picoseconds, a repetition ratefrom continuous wave to Megahertz or higher, and a time averagedintensity of less than approximately 50 MW/cm². The output beam has atime averaged output power of about 20 mW or more.

One embodiment of a broadband light source comprises one or more laserdiodes capable of generating a pump signal with a wavelength shorterthan 2.5 microns and a pulse width of at least 100 picoseconds. The oneor more laser diodes are coupled to one or more optical amplifiers,which are capable of amplifying the pump signal to a peak power of atleast 500 W. A first fiber is further coupled to the one or more opticalamplifiers, wherein the pump signal wavelength falls in an anomalousgroup-velocity dispersion regime of the first fiber, wherein the pumpsignal is modulated using a modulational instability mechanism in thefirst fiber, and wherein different intensities of the pump signal cancause relative motion between different parts of the modulated pumpsignal produced through modulational instability in the first fiber. Anonlinear element is coupled to the first fiber, and the nonlinearelement is capable of broadening the pump optical spectral width to atleast 100 nm through a nonlinear effect in the element.

In another embodiment, a mid-infrared light source comprises one or morelaser diodes comprising a wavelength and a pulse width of at least 100picoseconds. One or more optical amplifiers are coupled to the pumpsignal and are capable of amplifying the pump signal. Further, one ormore fibers are coupled to the optical amplifiers. In the fibers, thepump signal wavelength falls in the anomalous group-velocity dispersionregime for at least a fraction of the one or more fibers, and the pumpsignal is modulated using a modulational instability mechanism. Anonlinear element is coupled to the one or more fibers and is capable ofgenerating a super-continuum with a substantially continuous spectrumfrom at least the pump signal wavelength out to 2.6 microns or longerand wherein the nonlinear element introduces less than 10 decibels ofpower loss at 2.6 microns.

A further embodiment involves a method of generating broadband light bygenerating a pump signal, wherein the pump signal comprises a wavelengthshorter than 2.5 microns and a pulse width of at least 100 picoseconds.The method further comprises the step of amplifying the pump signal to apeak power of at least 500 W, modulating at least a fraction of the pumpsignal using a modulational instability mechanism, and broadening thepump optical spectral width to at least 100 nm using a nonlinear effect.

In yet another embodiment, a MIRFIL can use technologies that have beendeveloped for telecommunications. For example, the pump laser can be alaser diode followed by multiple stages of optical amplifiers. The pumpcan use continuous wave (CW) or quasi-CW light, which may comprisepulses broader than approximately 100 picoseconds. In a preferredembodiment, the mid-IR light generation may occur in an open loop offiber, preferably a fiber that transmits light into the mid-IR.Advantageously, only a short length of fiber can be used, such as lessthan about 100 meters, preferably less than about 20 m, and even morepreferably less than about 10 m. With this configuration, wavelengthscan be generated in the fiber beyond approximately 1.8 microns,preferably beyond approximately 2.2 microns, and even more preferablybeyond 2.5 microns.

In a particular embodiment, a MIRFIL can use a laser diode driven pumplaser that outputs CW or quasi-CW pulses (greater than approximately 100picoseconds) followed by a series of fibers, wherein the first length offiber can be made from fused silica and can be used to break the CW orquasi-CW light into pulses based on the modulational instability (MI) orparametric amplification effect, and then another length of mid-IRfiber, such as ZBLAN, fluoride, tellurite, or a semiconductor waveguidecan be used to broaden the spectrum, through the nonlinearity in themedium and a mechanism such as self-phase modulation. In a preferredembodiment, some curvature in the temporal domain can help to generatethe super-continuum by causing relative motion between the MI generatedpulses. Also, there can advantageously be exchange of energy between MIgenerated pulses through the Raman effect in the medium. The design ofsuch a MIRFIL can be that the MI-induced pulse break-up may occurprimarily in the first section, and the nonlinear spectrum generationmay occur primarily in the second section. In a preferred embodiment,the length of the fused silica fiber can be under 10 meters, and thelength of the mid-IR fiber can be less than 20 meters.

In another embodiment, super-continuum (SC) generation from the visibleor near-IR wavelength range can be accomplished using nanosecond pulsepumping. The SC generation can exemplary be initiated using modulationalinstability (MI). In a preferred embodiment, the seed for MI may arisefrom the amplified spontaneous emission from the optical amplifiers orfrom a near-IR light source, such as a laser diode. In a particularembodiment using fused silica fiber, the SC can cover the wavelengthrange substantially between approximately 0.8 microns to approximately2.8 microns. In another particular embodiment using ZBLAN fluoridefiber, the SC can cover the wavelength range substantially betweenapproximately 0.8 microns to approximately 4.5 microns. With control ofthe fiber loss from the material or from bend induced loss, as well withtailoring the composition of the fluoride fiber, the long wavelengthedge of the SC may be pushed out to 5.3 microns or longer. In apreferred embodiment, it may be valuable to add a wavelength conversionstage. In addition, it may be advantageous to have a pulse compressionstage following the MI pulse break-up.

In yet another embodiment, wavelength conversion into the mid-IRwavelength range can be achieved based on four-wave mixing (4WM) infibers. 4WM usually requires phase matching, and a new window for phasematching permits phase matching into the mid-IR. In a preferredembodiment, the phase matching wavelengths can be tuned by adjusting thefiber dispersion profile and tuning the seed wavelength in the near-IR.In a particular embodiment, a solid core or photonic crystal fiber canbe used with a tailored dispersion profile, a seed wavelength from alaser diode or a tunable laser in the near-IR can be used to convertlight from a near-IR pump to the mid-IR wavelength range.

In another embodiment, the power for the MIRFIL can be scaled up byusing a higher power pump laser, such as a cladding pumped fiberamplifier, a cladding pumped fiber laser or a solid state diode-pumpedlight source. Based on the damage threshold of the particular fiberemployed, the core size of the fiber can also be increased to increasethe power throughput and output power.

The fiber based mid-IR light source may be an enabling technology for anumber of applications. For example, the broadband mid-IR light sourcemay be useful for infrared counter-measures for aircraft protection.Also, the SC light source could be used in chemical sensing, fornon-contact or remote sensing of firearms, weapons, drugs. The SC sourcecould also be used for industrial chemical sensing, such as in advancedsemiconductor process control, combustion monitoring, or chemical plantprocess control. Other potential applications include bio-medicalimaging and ablation. Moreover, the broadband SC light source couldadvantageously be used in an optical coherence tomography configurationfor semiconductor wafer imaging or defect location. In addition, thebroadband light source could be instrumental for applications in thelast mile solution, such as fiber to the home, node, neighborhood, curb,premise, etc. More specifically, the broadband light source could enablewavelength division multiplexed or lambda passive optical networks.

Depending on the specific features implemented, particular embodimentsof the present invention may exhibit some, none, or all of the followingtechnical advantages. Various embodiments may be capable of coveringother wavelength ranges or multiple wavelength ranges. For example, SCgeneration can cover the visible wavelength range from approximately 0.4microns to 0.6 microns by using a dual pumping scheme. Some embodimentsmay be capable of generating bands of wavelengths rather a continuousrange of wavelengths, and the bands of wavelengths may also be tunableor adjustable.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, description and claims. Moreover,while specific advantages have been enumerated, various embodiments mayinclude all, some or none of the enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present invention andcertain features and advantages, thereof, reference is made to thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates a calculated group velocity dispersion for fusedsilica fiber (top), fluoride fiber (second from top), sulfide fiber(third from top) and selenide fiber (bottom).

FIG. 2 illustrates different positions of the pump and zero dispersionwavelength for (a) fused silica fiber; (b) fluoride fiber; and (c)chalcogenide fiber.

FIG. 3 illustrates modulational instability in the time domain.

FIG. 4 illustrates modulational instability in the frequency orwavelength domain.

FIG. 5 illustrates simulations of pulse propagation in fiber.

FIG. 6 illustrates simulations of pulse propagation in longer fiberlengths.

FIG. 7 illustrates the experimental set-up for a particular embodimentof the pump laser.

FIG. 8 illustrates a high power pump experimental configuration.

FIG. 9 illustrates the spectrum from high nonlinearity fiber with a zerodispersion wavelength of 1544 nm. (a) 3 m length of non-dried fiber; (b)5 m length of extra-dried fiber.

FIG. 10 illustrates: (a) Spectrum as a function of high nonlinearityfiber length following a ˜2 m length of SMF fiber. (b) Complete SCspectrum from ˜2 m SMF plus 15 cm of high nonlinearity fiber.

FIG. 11 illustrates: (a) Autocorrelation showing the pulse break-upthrough modulational instability after 3 m of SMF at 1 kW peak power.(b) Spectrum for same case with 1 kW peak power.

FIG. 12 illustrates the attenuation constant (dB/km) for different ZBLANfluoride fibers used in the experiments. (a) 1.25, (b) 1.75 and (c) 2.75micron cut-off wavelength.

FIG. 13 illustrates: (a) Comparison of fluoride super-continuum fordifferent fiber lengths following a ˜2 m length of SMF fused silicafiber; and (b) Spectrum from ˜5 m length of the second fluoride fiber(FIG. 12 b) following ˜2 m length of SMF fused silica fiber.

FIG. 14 illustrates: (a) the long wavelength side of the super-continuumspectrum from different lengths of the third fluoride fiber (FIG. 12 c)following an approximately 1 m length of fused silica SMF fiber. Thelong wavelength edge reaches to ˜4.5-˜4.6 microns; (b) Power evolutionof the spectrum from ˜2 m length of the third fluoride fiber (FIG. 12 c)following an approximately 1 m length of fused silica SMF fiber; and (c)overall calibrated spectrum from ˜7 m of the third fluoride fiber (FIG.12 c) following an approximately 1 m length of fused silica SMF fiber.

FIG. 15 illustrates the calculated modulational instability gain for 3.5kW at 1630 nm and 0, 1, 2 and 3.5 kW at 1635 nm.

FIG. 16 illustrates a generalized model for super-continuum generationfrom quasi-CW or CW pumping. Note that some, all, or none of theillustrated boxes may be involved in SC generation. Further, other boxescan also be added within the scope of the disclosure.

FIG. 17 illustrates cascaded Raman wavelength shifting data from (a)WS#884 Corning fiber measured with an optical spectrum analyzer; (b)WS#884 Corning fiber at higher power and measured with a spectrometer.

FIG. 18 illustrates the phase mismatch in a fluoride fiber with zerodispersion wavelength near 1.7 microns.

FIG. 19 illustrates an exemplary experimental configuration for testingfour-wave-mixing.

FIG. 20 illustrates an exemplary spectral fingerprinting system blockdiagram. Note that some, all, or none of the illustrated boxes may beinvolved in spectral fingerprinting. Further, other boxes can also beadded within the scope of the disclosure.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Mid-IR light can be generated based on super-continuum in fused silicafibers and mid-IR fibers. Nonlinear waveguides other than fibers canalso be used to generate the super-continuum. In one embodiment, SC hasbeen demonstrated experimentally from ˜0.8 to ˜4.5 microns in ZBLANfluoride fibers and from ˜0.9 to ˜2.8 microns in high-nonlinearity(HiNL) fused silica fiber. The SC originates for laser diode pumping,and modulational instability (MI) initiated SC generation leads to asignificant simplification by reducing or eliminating the need forexpensive, complicated, mode-locked lasers. In another embodiment, threeorders of cascaded Raman wavelength shifting (CRWS) can be observed insulfide-based chalcogenide fibers below the damage threshold. In oneparticular embodiment, the pump source comprises a laser diode followedby several stages of erbium-doped fiber amplifiers, in some cases alsoincluding a mid-stage modulator. Since in this embodiment the SC or CRWSoccurs in meters to 10's of meters of fiber, the entire mid-IR lightsource can be compact, lightweight, inexpensive and rugged. Althoughparticular experimental conditions are described in the following, otherconfigurations, materials and fiber types can be used within the scopeof the invention.

Fiber Dispersion can Determine SC Vs. Cascaded Raman Shifting

To organize and explain the experimental results in various types offibers tested, a theoretical framework is first established. Variousnonlinear processes are observed in fibers, included CRWS and MI. Inturn, MI can give rise to the generation of broadband SC. Whether CRWSor SC occurs first in a fiber depends on the wavelength of the pump orthe shifted pump with respect to the zero dispersion wavelength Λ₀. Whenthe pump is at a wavelength shorter than the zero dispersion wavelength(so-called normal dispersion regime), then CRWS can be first observed.When the pump is at a wavelength longer than the zero dispersionwavelength (so-called anomalous dispersion regime), then MI and SC canbe first observed. When the pump lies in the normal dispersion regime,it can experience CRWS, but when the cascaded Raman order shifts intothe anomalous dispersion regime, then MI and SC can occur. Thus, tounderstand the nonlinear spectrum generated in fibers, the position ofthe zero dispersion wavelength can indicate the expected behavior.

In the Raman effect, a strong pump beam coupled into the fiber can shakethe glass matrix, which emits vibrational mode (so-called opticalphonons), and then can provide gain to longer wavelengths. The Ramaneffect can be self-phase matched, and hence the process does notgenerally require tuning and it can be more-or-less independent ofwavelength (the gain coefficient does scale inversely with wavelength,however). One attribute of the Raman effect is that a number of opticalphonons can be emitted, or the wavelength can be shifted down through acascaded Raman process sequentially to longer and longer wavelengths.This emission of a plurality of phonons to shift more than one Ramanorder is the CRWS phenomena that can be observed in the normaldispersion regime.

Either pure continuous wave (CW) light or quasi-CW light, such asnanosecond or longer pulses, are generally unstable when launched in theanomalous dispersion regime. In particular, the interaction between thenonlinearity and anomalous dispersion can break the quasi-CW inputs intoa train of solitons in a process called modulational instability. MI canbe considered as a parametric four-wave-mixing process in which thenon-linearity explicitly enters the phase matching condition. Note thatMI for a single pump wavelength generally phase matches in the anomalousdispersion regime. When MI occurs the peak powers reached in the fibercan be much higher than the powers launched in the quasi-CW light, sincethe quasi-CW background is usually compressed into short pulses.Further, a curvature in the pulse in time or a range of intensities canlead to collision and energy exchange between the MI-generated pulses,which can be advantageous for SC generation.

For pumping in the anomalous dispersion regime, the combined effects ofMI and stimulated Raman scattering can lead to SC generation. MI cancause the break-up of the CW light into short temporal pulses such thatthose nonlinear phenomena that normally occur for pulsed pumpingconditions can also contribute to the SC generation. In contrast, fornormal dispersion pumping, CRWS generally occurs first in the fiber,since MI does not generally phase match for a single pump wavelength. Asthe higher Stokes orders fall into the anomalous-dispersion regime, MIcan occur and lead to SC generation.

Because of the relevance of the group velocity dispersion (GVD) fordetermining the nonlinear behavior observed, a brief review is providedof fiber dispersion. GVD arises because different frequencies of lighttravel at different speeds in an optical fiber. The total GVD in thefiber is generally the sum of the material dispersion and the waveguidedispersion D_(total)≈D_(m)+D_(w). The zero dispersion wavelength of afiber Λ₀ corresponds to the wavelength where the total dispersioncrosses through zero.

The waveguide dispersion arises because the mode distribution betweenthe fiber's core and cladding changes with wavelength. In a solid corefiber, the waveguide dispersion is usually negative, and, therefore, cangenerally shift the zero dispersion wavelength to longer wavelengths. Itshould be noted that in microstructure fibers, the zero dispersionwavelength can be shifted to any desired wavelength. Therefore,microstructure fibers can be useful for matching the zero dispersionwavelength to the laser wavelength when the laser wavelength fallsoutside of the usual telecommunications window between ˜1.3 and ˜1.6microns.

The material dispersion curves for different fibers tested in theexemplary experiments are illustrated in FIG. 1. In particular, thecurves are calculated from published index-of-refraction for differentglasses. The index is given analytically and the dispersion, which isproportional to the second derivative of the index, is calculatednumerically. For fused silica fiber 110, the zero dispersion wavelengthcan be close to 1300 nm, corresponding to the material dispersion zero.Using dispersion shifted fibers, the zero dispersion wavelength can beshifted to longer wavelengths. For fluoride fibers 120, the zerodispersion wavelength can be calculated to be approximately 1620 nm.Note that the dispersion slope for the fluoride fiber appears to be muchflatter than for fused silica fiber. The calculated dispersions forchalcogenide fibers, such as the sulfide fiber 130 and selenide fiber140 show a zero dispersion wavelength beyond 5 microns.

Based on the theoretical discussion above and the exemplary use of apump wavelength near ˜1553 nm, the behavior for different fiber typesillustrated in FIG. 1 can be predicted. In fused silica fiber,MI-initiated SC generation is expected (FIG. 2 a). For the fluoridefiber, at first cascaded Raman wavelength shifting is expected 220, butafter one or two Stokes orders SC should be generated (FIG. 2 b).Finally, for the chalcogenide fibers, CRWS is expected out to beyond 5microns (FIG. 2 c).

Because of the relevance of modulational instability (MI), a briefreview is also provided next. MI can be a parametric four-wave-mixingprocess in which the anomalous group velocity dispersion andnonlinearity generally work together to turn CW or quasi-CW light intoshort pulses. MI can lead to a significant system simplification in SCgeneration because the CW or quasi-CW light can evolve into narrow andhigh peak power soliton pulses, thus reducing or limiting the need formodelocked lasers for SC generation. As a simple illustration, FIG. 3shows the time domain evolution of quasi-CW pulses in the anomalousdispersion regime. Noise perturbations (such as different longitudinalmodes from the laser diode or the amplified spontaneous emission fromthe optical amplifiers) can cause temporal perturbations to grow 310,leading to the formation of a train of soliton pulses 320 through MI. Asan alternative technique, a seed laser at a wavelength separated fromthe pump can be used to initiate MI. Then, the combined effect of MI andstimulated Raman scattering (SRS) can lead to the red-shifted pulses tomove with respect to the blue-shifted pulses, causing a further increasein the peak intensity 340. If the intensity is nearly flat, then a trainof solitons could be generated, but these pulses may not move withrespect to each other. If the pulses have a non-uniform temporal profileor if an intensity modulation is introduced on the pump light, then thenon-uniform pump intensity can lead to movement of the MI-generatedpulses with respect to each other. Then, energy exchange can occurbetween the pulses through the Raman effect.

FIG. 4 illustrates the spectral domain evolution corresponding to thetime-domain description of FIG. 3. The original laser frequency is givenby 410. MI can lead to the generation of sidebands on the Stokes andanti-Stokes side of the original laser frequency 420. Although only oneset of sidebands are illustrated, in general there can be severalsideband frequencies generated approximately symmetrically around theoriginal laser beam. Then, when MI and SRS interact, SRS leads to atransfer of energy from the short wavelength side to the long wavelengthside 430.

The dynamics of the collision of soliton pulses can be quitecomplicated, and it may be easier to illustrate through computersimulations. As an example, FIGS. 5 and 6 show simulations of thebreak-up of quasi-CW pulses and then the onset of SC generation. FIG. 5shows the initial break-up of the quasi-CW pulse 510 into solitons 540,and then the shift in energy to the longer wavelength side through SRS(left side 550 is time domain, right side 560 is frequency domain). FIG.6 shows the time-domain collision process further down the fiber, as theonset of continuum generation is seen on the computer. The red-shiftedpulses 670 travel through the blue-shifted pulses 680 because of theanomalous GVD, and then the red-shifted pulses 670 can rob energy fromthe blue-shifted pulses 680 through SRS (also sometimes called thesoliton self-frequency effect). As FIG. 6 illustrates, relative motionbetween pulses can lead to collision and the consequent exchange ofenergy between pulses. Thus, a non-uniform temporal profile or anintensity modulation of some sort may be required to cause high peakpower pulses that lead to SC generation. This complicated collisionprocess can give rise to narrow, high peak power pulses 670, 650, 660,which are responsible for the SC.

The simulations of FIG. 6 also show that asymmetric pulses 650,660 canarise from the soliton collision process. The asymmetry in the resultingpulses can also lead to an asymmetric spectrum. For instance, it isknown that self-steepening of pulses can lead to a larger frequencybroadening on the blue-side of the spectrum (i.e., the short wavelengthside of the spectrum). Moreover, asymmetric temporal profiles of pulseslead to asymmetric spectral broadening through self-phase modulation. Itshould also be noted that the four-photon process is in energy, ratherthan wavelength. Thus, two pump photons can lead to the generation ofStokes and anti-Stokes photons.

Whether CRSW or MI is first seen in the fiber generally depends on thethreshold for the different nonlinearities. For instance, in fusedsilica fiber the MI threshold can be ˜5× less than the Raman threshold.There are generally two components for the Kerr nonlinearity n₂: ˜⅘ ofn₂ is electronic in nature (instantaneous and arising from the UVresonances), while ˜⅕ of n₂ is from Raman-active vibrations (imaginarypart of this is the Raman gain coefficient). Whereas MI usually benefitsfrom the full n₂, CRWS usually only benefits from the Raman-activevibrations. However, the Raman effect is generally self-phase matched,while MI usually requires phase matching. Moreover, MI phase matches fora single pump wavelength typically in the anomalous group velocitydispersion regime. Therefore, in the normal dispersion regime CRWSusually has a lower threshold and MI does not usually phase match for asingle pump wavelength. On the other hand, in the anomalous dispersionregime MI can have a lower threshold, and it can initiate the pulsebreak-up that gives rise to SC generation through a combination of SRS,self-phase modulation and other nonlinearities.

Although particular examples of physical phenomena have been describedfor MI or CRWS and subsequent SC generation, other techniques are alsopossible for MI, CRWS and SC. As one particular example, the use ofmultiple pump wavelengths can lead to MI and SC even in the normaldispersion regime. As another example, the pulse break-up through MI maynot need to be as thorough as shown in the simulations to lead to SCgeneration. Thus, many other combinations of physical phenomena can leadto MI, SC and CRWS.

Pump Laser for Testing Nonlinear Shifting in Various Fibers

As one particular embodiment, experiments have been conducted onnonlinear wavelength shifting in different fiber types, including fusedsilica (high-nonlinearity fiber—HiNL—as well as dispersion compensatingfiber), chalcogenide fibers (arsenic-tri-sulfide), and fluoride (heavymetal fluoride ZBLAN—ZrF₄—BaF₂—LaF₃—AlF₃—NaF). For the experimentalembodiment, the pump set-up uses a pulsed laser diode (˜1.8 nsec pulsesnear 1553 nm) followed by several stages of erbium-doped fiberamplifiers. Peak powers up to ˜3 kW can be generated by altering theduty cycle of the laser diode from a few hundred kHz down to 5 kHz.

The experimental configuration for the pump laser used to test thedifferent fibers is shown in FIG. 7. Light originates from a distributedfeedback (DFB) laser diode 705 at 1553 nm. This light is amplified in alow-noise pre-amplifier 710, which comprises an erbium-doped fiberamplifier (EDFA). The light is boosted in a power amplifier 740 at thelast stage. In between, filters 720,735 and modulators 715 are used tocontrol the amplified spontaneous emission (ASE).

The set-up 700 emulates a Q-switched laser system. Although the laserdiode cannot easily be Q-switched, the EDFAs have a long upper statelifetime and store up energy between pulses. Thus, a low duty cycleduring which the laser diode is turned on can lead to a larger energyper pulse. However, an ASE problem arises because when the laser diodeis off, the optical amplifier continues to be pumped and ASE is emittedby the EDFAs. This ASE leads to inefficiency because it can deplete someof the energy from the power amplifier, which ideally would store upmore energy before the next laser diode pulse passes.

To solve this problem, one solution that can be used is to block the ASEduring the times that the laser diode is off. As a starting point, theexperimental configuration of FIG. 7 can be used to reduce the ASEbackground. A fiber pigtailed modulator 715 can be placed between thepre-amplifier 710 and the power amplifier 730,740. The modulator windowcan be synchronized to the laser diode drive 760, and the delay to thepre-amplifier EDFA can be compensated by using a variable electricaldelay line 755. The modulator 715 is placed initially at the mid-stagepoint to optimize the noise figure of the overall amplifier. Also, alow-power EDFA 730 is added after the modulator 715 to compensate forthe modulator insertion loss. A tunable spectral filter 735 is also usedto limit the out-of-band ASE entering the power amplifier 740. Finally,the light is coupled to the high-power EDFA stage 740. As analternative, the modulator can be eliminated by using narrow-band, fibergrating based filters to minimize the effect of the ASE. The temporalmodulator is used in this particular embodiment. However, there are manyother embodiments and methods of controlling the level of ASE from theamplifier. As another example, narrow band filters or add/dropmultiplexers could be used to control the ASE contribution. In otherembodiments, the length of gain fiber and the direction and number ofpumps could be optimized to minimize the level of ASE.

A second problem that may arise in the last stage power amplifier isnonlinear fiber effects, which then can limit the useable power from thepump system. The last amplifier stage comprises as an example two WDMcouplers (for coupling in and removing any residual 980 nm pump)surrounding a highly-doped, large core size, single spatial mode EDFAgain fiber. The gain fiber is selected in this instance to minimize thenonlinear limitations in the final amplifier stage. For example, thehigh doping level means that a short fiber length can be used, and thelarge core size means that the intensity is kept as low as consistentwith a single spatial mode. In this particular embodiment, the poweramplifier uses a ˜1.2 m length of gain fiber, and a forward pump is usedin addition to a backward pump. With this set-up, the peak power for 5kHz repetition rate approaches ˜2.5 kW without any significant nonlineareffect in the amplifier.

One aspect of using nanosecond pulses with peak powers up to ˜2-3 kW isthat the average power can be scaled up by increasing the repetitionrate and using larger lasers, such as cladding pumped fiber amplifiersor lasers. For example, average powers in the range of 1 kW to 15 kW areavailable from commercially available cladding-pumped fiber lasers.Moreover, fiber lasers can be modulated or Q-switched or a modulator canbe placed after the fiber laser to generate nanosecond pulses relativelyeasily. Cladding pumped fiber lasers can operate at a number ofwavelengths. For instance, ytterbium-doped cladding pumped fiber lasersoperate near 1 micron, erbium-doped cladding pumped fiber lasers operatenear 1.55 microns, and thulium-doped cladding pumped fiber lasersoperate near 2 microns. Alternatively, the laser could a solid statelaser or a diode-pumped laser. Although a few examples of high powerlasers are mentioned above, many other lasers can be used consistentwith the scope of this disclosure.

In one particular embodiment, a high power pump can comprise a seedlaser diode that may be modulated followed by several stages ofamplifiers that are single-mode fibers or cladding pumped fiberamplifiers. For example, the first stage pre-amplifier can be asingle-mode fiber, such as an erbium-doped fiber amplifier. Then, thepower amplifier can comprise one or more stages of cladding-pumped fiberamplifiers. In a cladding pumped fiber amplifier, the pump propagatesthrough a fiber cross-sectional area that is typically larger than asignal cross-sectional area. A cladding-pumped fiber amplifier maycomprise a fiber with a core for a signal and the pump that zig-zagsthrough the signal and provides gain. As one particular example, thecladding-pumped fiber amplifier may be a double clad fiber, with thesignal propagating through the core and the pump propagating through theinner cladding. For high gain systems, the cladding-mode fiber amplifiermay also be a large mode area fiber, which generally means that the coreis large enough to support several modes. The cladding-pumped fiberamplifier can be doped with erbium or a combination of erbium andytterbium. Spectral and/or temporal filters may be advantageously usedbetween different amplifier stages to control the level of amplifierspontaneous emission. Also, particularly in the last few stages ofamplification, it may be advantageous to counter-propagate the pump fromthe signal, thereby reducing the nonlinear effects in the amplifier.

In one preferred embodiment, the pump laser 800 can be a modulated laserdiode 820 followed by a parametric amplifier 860. For example, FIG. 8illustrates a cladding pumped optical parametric amplifier system 800that can generate peak powers in excess of 10 kW with nanosecond pulses.The configuration comprises laser diodes 810,820, cladding pumpedytterbium-doped fiber amplifiers 830, and an optical parametricamplifier 860. The top arm 870 of FIG. 8 corresponds to the pump, whilethe bottom arm 880 corresponds to the signal seed. In particular, alaser diode 810 (either Fabry-Perot or distributed Bragg reflector—DBR)launches the pump light at approximately 1064 nm. This is first passedthrough a pre-amplifier, which comprises a single-mode fiber doped withytterbium. Then, the output of the pre-amplifier is sent to a poweramplifier, which comprises a cladding-pumped, multi-mode ytterbium-dopedfiber amplifier 850. The power amplifier is purposely made with a largecore fiber so as to enable high power amplification (i.e., large gainvolume) while minimizing nonlinear effects (i.e., large effective area).

The signal seed originates in the lower arm 880 from a 1550 nm laserdiode 820, such as a DBR, DFB or Fabry-Perot laser diode. The light fromthe seed laser diode 820 is pre-amplified in a single-mode, EDFA 840 inthis embodiment. Then, the about 1550 nm seed is boosted in a poweramplifier, which in this preferred embodiment is an optical parametricamplifier 860 (OPA). The OPA 860 comprises a periodically-poled lithiumniobate crystal in a preferred embodiment. The OPA crystal can be inlength between several millimeters to several centimeters. The pump at1064 nm 850 and the seed at 1550 nm 880 are made collinear through theOPA crystal 860, and the 1550 nm light is power amplified through theOPA process. Although one example of the OPA is discussed here, manyother power boosting methods can be used within the scope of thisdisclosure.

Another aspect of the laser is that it is relatively simple to modulatethe mid-IR light or the SC light. Rather than implementing a mid-IRmodulator or a very broadband modulator, the modulation can be done onthe pump laser. For example, the pump laser can be modulated directly(i.e., modulating the pump laser diodes or the power supply) orexternally modulated (i.e., place a modulator after the pump laser).Then, in the SC generation process, the modulation is transferred to theentire broadband spectrum. In other words, all of the optical processingcan be performed at the pump wavelength, and then the light can beshifted to other wavelengths in the last step. This approach isparticularly attractive when the pump laser is at a telecommunicationswavelength, since many modulator technologies are available for telecomwavelengths.

The pump laser described above is just one embodiment of the pump laser,but many other pump lasers can be used. For example, the pump laser canbe a cladding-pumped fiber amplifier or laser, a diode-pumped solidstate laser, or either of the lasers followed by a cascaded Ramanwavelength shifter. The cascaded Raman wavelength shifter may be an openloop piece of fiber, or cascaded resonators formed by placing gratingson one or more ends of the fiber. Thus, many different configurationsfor the pump laser can be used consistent with the SC or wavelengthconversion process.

To generate light in the mid-IR, one exemplary strategy is to test anumber of different kinds of fibers. The starting point may be to usefused silica (SiO₂) fiber, since it is of the highest quality andbecause it is the best characterized fiber. For example, fused silicafiber is the basis of most fiber optics communications. In addition,fused silica fiber has among the highest damage threshold (˜50 MW/cm²),which means that it can extend to the highest output powers. Moreover,the physics of fused silica is well understood and the parameter valuescan be measured carefully, permitting detailed understanding of themechanisms behind SC generation and wavelength conversion. Furthermore,the dispersion can be tailored in fused silica fiber, different types offibers can be spliced together to create a particular dispersionprofile, and more exotic fiber geometries, such as photonic crystalfibers or microstructure fibers, can be implemented in fused silica.

Although fused silica is the starting point, the transmission of fusedsilica is limited in the mid-IR. Therefore, with the understandinggained from fused silica, the next step in the strategy can be to usefibers that transmit in the mid-IR. One attractive candidate for mid-IRtransmission is ZBLAN fluoride fibers. These fibers have been madesingle and multi-mode for over 25 years. They have been used extensivelyin telecommunications, for example as praseodymium-doped fiberamplifiers and erbium-doped fluoride fiber amplifiers. The fluoridefibers also have relatively low loss and relatively high damagethreshold (depending on the impurity concentration, typically between˜10 to ˜20 MW/cm²). Furthermore, by adjusting the composition of thefluoride fibers, the low-loss transmission band can be extended out tobetween ˜4.5 microns and ˜5.5 microns.

Beyond ZBLAN fluoride fibers, other fibers or waveguides could also becandidates for mid-IR light generation. As one example, tellurite fibers(TeO₂) can be used as mid-IR fibers. Tellurite glass compositions showenhanced Raman scattering behavior. By optimizing these oxide glasscompositions with heavy-metal-oxides, fiber can be made that have highnonlinearity with transparency in the mid-IR wavelength range. Moreover,the tellurite fibers have been measured to have a damage threshold of˜13 GW/cm². Other examples of mid-IR fibers include chalcogenide fibers(telluride, sulfide, selenide, as particular examples), sapphire fibers,AgBrCl fibers, etc.

As another example, silicon or other semiconductor waveguides could beused to generate mid-IR light. Silicon waveguides are expected totransmit light over the entire mid-IR wavelength band. Also, by makingcurves or S-type (i.e., waveguide going back and forth three times on achip), relatively long lengths (i.e., several or tens of centimeters) ofwaveguide can be used. The use of silicon or other semiconductorwaveguides is particularly effective if a pre-stage of fused silicafiber is first used to initiate the MI (discussed further below). Inthis case, the semiconductor waveguide serves primarily as thetransparent, nonlinear element to lead to spectral broadening.

Although particular fiber types or waveguide structures have beendescribed for advantageously generating super-continuum, othermaterials, compositions and guided wave structures can be usedconsistent with the disclosure.

SC Out to Mid-IR in Fused Silica Fiber

The exemplary experiments use different lengths of fused silica fiber,which are a series of high-nonlinearity (HiNL) fibers made by Corning.The fibers have a zero dispersion wavelength between ˜1500 nm and ˜1950nm. Some of the fibers had extra drying steps to remove to the extentpossible the OH content, using steps that are commercially done forSMF-e Corning fiber. Lengths ranging from 1 to 13 m provide the broadestwidth of super-continuum in these particular experiments. Although theseparticular lengths were used in the experiments, other lengths can beused within the scope of the disclosure.

As an example of the SC spectrum from fused silica fiber, FIG. 9 a showsthe spectrum obtained 920 at ˜2.4 kW peak power in a 3 meter length ofHiNL fiber. For this particular fiber no additional drying steps weretaken to reduce the OH content, so a large OH absorption line may beexpected around 2.7 microns. At this pump power, the spectrum 920 isseen to stretch from ˜0.85 nm to ˜2600 nm (2.6 microns). The features925 around 1553 nm are the residual pump from the laser diode, and thepeaks near 1530 nm are due to the ASE from the EDFA's. A fairly smoothspectrum 920 is observed over the large spectral range. One reason forthe edge of the spectrum around ˜2600 nm might be that the edge of thewater absorption line is responsible for the cut-off. Another reasonmight be that at these long wavelengths the modes are weakly guided,and, hence, they are much more susceptible to bend induced loss.

To reduce the effects of bend-induced loss, the fiber can be laid outloosely. To reduce the effects of water absorption, the fibers can bedried using techniques used in commercial fibers, such as Corning'sSMF-28e fiber or Lucent's (now OFS Fitel's) All-Wave fiber. To test thishypothesis, a new batch of HiNL fibers were made that were treated usingthe extra drying steps. As an example, FIG. 9 b illustrates the SCspectrum from 5 meter length of extra-dried, HiNL fiber with zerodispersion wavelength around 1544 nm. The spectra 930, 940, 950 areshown as a function of different pump powers, and the spectrum isobserved to reach out to ˜2700 or ˜2800 nm. Therefore, the extra dryingsteps to enable the expansion to the longer wavelength side by about 100to 200 nm. The HiNL fibers used in these experiments have a nonlinearityabout 9 times larger than standard SMF-28 fused silica fiber.

The edge of the SC spectrum could potentially be due to the vibrationalabsorption in the fused silica glass. If the edge of the spectrum islimited by the fiber loss, then it would be consistent that the spectrummight extend to longer wavelengths if the fiber length were to bereduced. However, sufficient fiber length is required to generate thefull spectrum as well. In other words, there is a minimum lengthrequired to generate the spectrum, but then further propagation in thelossy fiber only reduces the long wavelength edge of the spectrum.

To understand this fiber loss limited spectrum further, a series ofexperiments were conducted at a pump peak power of ˜3 kW. First, an ˜2 mof standard single-mode fiber (SMF-28) is used to cause the nanosecondpulses to break up through MI. In fact, at these power levels SC can bealready generated in the SMF fiber, with a reach out to ˜2500 nm. Then,the output of the SMF fiber is coupled to different lengths of HiNLfiber with a zero dispersion wavelength near 1544 nm. The data 1000 isillustrated in FIG. 10 a. The fiber length is varied from 20 m 1020 to10 m 1030 to 5 m 1040 to 1 m and shorter 1050. As the fiber length isreduced, the long wavelength edge of the SC appears to push out tolonger wavelengths. The levels of the SC are also plotted correctlyrelative to each other. In other words, as the fiber length is reduced,not only does the long wavelength edge appear to push out, but also thelevel of the SC can increase. The broadest spectrum is reached withabout 15 cm of HiNL fiber, where the spectrum reaches beyond ˜2800 nm.The total spectrum 1010 from the ˜2 m of SMF plus 15 cm of HiNL isplotted in FIG. 10 b.

The spectra of FIG. 10 illustrates why others performing SC experimentsmay not have reached out as far in wavelength as the experimentsdescribed. Since the generation appears to be loss limited at the longwavelength side, the fiber length should be long enough to generate thespectrum through nonlinear effects, but not longer than that. In otherwords, optimizing the length of the fiber can be a procedure that canhelp to generate spectra as far as possible on the long wavelength side.

The data of FIG. 10 also suggests a strategy or recipe for generatingthe SC. First, a pre-stage of standard single mode fiber (SMF) can beused to break-up the pulses through modulational instability. AlthoughSMF is used in this example, the fiber can be any number of fibers thatexhibit MI, such as fibers who fall into the anomalous dispersion regimeat the pump wavelength. To illustrate the pulse break-up, FIG. 11 ashows the autocorrelation 1100 of the pulse at the output of a 3 mlength of SMF for 1 kW peak power, and FIG. 11 b shows the spectrum 1110at the output of the same fiber. As can be seen, wavelength sidebandsare generated by MI, which causes the pulse to have undulations withpulse widths down into the sub-picosecond range. Different power levelscan experience break-up in different fiber lengths. For instance, if thepeak power is closer to ˜3 kW, then the optimal length for pulsebreak-up is closer to ˜1 m of SMF. Thus, in the pre-stage fiber it isdesired to have a break-up of the CW or quasi-CW pulses into shorterpulses through MI, but not the complete generation of the SC spectrum.In other words, the pre-stage fiber and the MI phenomena serve toemulate the picosecond or femtosecond pulses that are normally used togenerate SC, but the natural physics of the fiber can accomplish thepulses without the need for expensive and complicated modelockingschemes.

The second step of the strategy or recipe can be to use a nonlinearelement with at least partial transparency over the wavelength range ofinterest to broaden out the spectrum and to smoothen the spectrum into aSC. As one example, the dominant mechanism in the second stage can beself-phase modulation, where the nonlinearity for the high peak powerpulses leads to spectral broadening. In addition, the Raman effect canalso be effective in transferring energy from the short wavelength sideto the long wavelength side, or more generally from shorter wavelengthsto longer wavelengths. In the case of fused silica fiber, the secondstage can be a relatively short length of HiNL fiber, as shown in FIG.10. Alternately, the fiber can be a ZBLAN or fluoride fiber that canpermit generation of light further out to closer to ˜4.5 microns. Otherexamples of nonlinear elements that can be used in the second stageinclude chalcogenide fibers, silicon waveguides, tellurite fibers, andother semiconductor waveguides. Alternatively, hollow core fibers can beused, where a nonlinear material, such as CS₂ or nonlinear gasses, canbe used to fill the hollow core.

Although a two stage strategy or recipe is given as an example, moresteps can be used to optimize the SC generation. For example, the firststage can be a set of fibers spliced or coupled together to achieve aparticular dispersion profile. In one preferred embodiment, the fiberscan be coupled to achieve a dispersion decreasing or dispersionincreasing profile. Moreover, a number of stages of the nonlinearelement can be used. In one preferred embodiment, the transparencyregion can be expanded in subsequent stages. In another embodiment,single mode as well as multimode fibers can be used in combination toobtain a high output power from the SC generation.

Although these experiments suggest that fused silica can generate SC outto ˜2.8 microns, the composition of fused silica can be altered topotentially achieve a wider wavelength range. As one particular example,fibers could be made from synthetic fused silica. For synthetic fusedsilica, there is a drop in transmission between ˜2.6 to ˜2.8 microns,which is probably due to the water absorption (OH absorption). Thetransmission through this wavelength range could be increased by usingextra drying steps to minimize the OH content. Note, however, that therecan be a transmission window between approximately 3 and 4 microns.Thus, with an appropriate fused silica composition, it may be possibleto generate SC out to ˜3.6 to ˜4 microns. This is just one example ofvarying the composition, but other compositions of fused silica couldalso be advantageous for SC generation into the mid-IR.

SC Generation in Fluoride Fibers

For mid-IR generation, fibers that have lower loss than fused silicainclude chalcogenide fibers, tellurite fibers, and fluoride fibers. Oneof the more mature of the fluoride fibers is the heavy metal fluorideZBLAN (ZrF₄—BaF₂—LaF₃—AlF₃—NaF). One advantage of the fluoride fiber isthat the loss coefficient can be more than two orders-of-magnitude lowerthan chalcogenide fibers over the wavelength range between ˜2-5 μm. TheRaman gain coefficient can be about ˜2-3× larger than in fused silicafiber. Moreover, the peak of the Raman gain falls at ˜600 cm⁻¹, andfluoride fibers tend to be more mature technology with higher laserdamage thresholds and no evidence of photo-darkening. For example,Alcatel and others made erbium-doped amplifiers and praesodynium dopedamplifiers based on ZBLAN fiber in the 1980's and 1990's.

Three lengths of ZBLAN fluoride fiber were obtained for exemplaryexperiments of SC generation. The first fiber is 45 m long with a corediameter of ˜5.7 microns and a cladding diameter of 125 microns and acut-off wavelength of ˜1.25 microns. The second fiber is a 85 m lengthof fiber with a core diameter of ˜6.5 microns and a cut-off wavelengthat ˜1.75 microns. A third fiber is ˜20 m long with a core diameter of ˜7microns, a cladding diameter of 125 microns, and a cut-off wavelength of˜2.75 microns (the longer cut-off is achieved by using a highernumerical aperture of ˜0.3). For all these fibers the loss between 1.25and 2.7 microns can be less than 10 dB/km (0.01 dB/m). There is a losspeak around 1 micron and another loss peak centered around 2.9 microns,and at these peaks the loss is between 30-50 dB/km. For the third fiber,the attenuation out to 4 microns is measured to be under 1 dB/m, and theattenuation beyond 4 microns is 1 dB/m at 4.3 microns, 2.25 dB/m at 4.5microns, and 8 dB/m at 4.8 microns. Based on the experience from fusedsilica fiber, SC generation should advantageously have a long wavelengthedge out to where the fiber has a loss of ˜1 dB/m to ˜2 dB/m. Therefore,the SC generation may be able to reach out to ˜4.5 to ˜4.6 microns inthe ZBLAN fibers.

The loss spectra measured over a limited wavelength range is shown inFIG. 12 for the three fibers measured to date. As the cut-off wavelengthincreases, the loss at the longer wavelengths appears to decrease. Thismay indicate that the loss at the longer wavelength arises at least inpart from bend induced loss. The rule-of-thumb for bend induced loss isthat the fiber can be well guided at least up to a wavelength that is˜1.5 times the cut-off wavelength. Based on this rule, the first fiber1210 should have minimal bend induced loss up to at least 1.9 microns,the second fiber 1220 should have minimal bend induced loss up to atleast 2.63 microns, and the third fiber 1230 should have minimal bendinduced loss up to at least 4.2 microns.

As the pump power is increased in the fluoride fiber, Raman wavelengthshifting is first observed experimentally. Then, SC generation occursafter the Raman order crosses to the long wavelength side of the zerodispersion wavelength. For the 85 m length of fluoride fiber of FIG. 12b, the SC spectrum with an input peak pump power of ˜2.5 kW stretchesfrom ˜850 nm to ˜3600 nm, and over the mid-IR region the spectraldensity ranges from −18 dBm/nm to −30 dBm/nm. As another example, in 40m of the first fiber of FIG. 12 a, the spectrum is found to reach onlyout to ˜3050 nm. The peak power launched in this case was ˜3.5 kW. Themagnitude of the long wavelength edge of the spectrum does appear to becorrelated with the shorter cut-off wavelength in this fiber. In otherwords, the longer the cut-off wavelength, the further that the longwavelength side of the SC spectrum extends out to.

One main difference between the fused silica SC and the fluoride SC isthe wavelength range expected. Whereas the glass transmission in fusedsilica would appear to limit the SC range to below 3 microns, because ofthe low loss in the fluoride fibers out to approximately 5 microns, theSC can continue to wavelengths longer than 3 microns. Moreover, sincethe dispersion slope is less in the fluoride fibers compared to fusedsilica (FIG. 1), the MI bandwidth for phase matching can be much larger,giving rise possibly to broader bandwidth SC generation.

The hypothesis is that the sharp wavelength edge observed in theexemplary ZBLAN fluoride fiber SC experiments arise from fiber bendinduced losses. i.e., As the wavelength increases, the mode diameterincreases and more of the mode penetrates into the cladding and isweakly guided. Several data points support the hypothesis of thespectral edge arising from the bend induced loss in the fiber. First,when the fluoride fiber was wound on a ˜8 inch spool, the edge of thespectrum reached to ˜3400 nm. When the same fiber was loosely laid in adrum, the spectral edge shifted toward longer wavelengths out to ˜3600nm. Second, the bend induced loss is measured at 3.3 microns. For a benddiameter of 50, 100 and 200 mm, the percent loss at 3.3 microns is 85%,3% and 1%, respectively. Therefore, the SC in the ZBLAN fluoride fibershould cover a wider range of the mid-IR when the bend induced is bettercontrolled.

Using a Fused Silica Pre-Stage Before the Fluoride Fiber

By using an appropriate length of fused silica pre-stage before thefluoride fiber, the length of fluoride fiber can be reduced and thespectral extent can be optimized. As an example, consider the secondfluoride fiber (specifications in FIG. 12 b). In the above describedexperiment, an approximately 85 m length of fiber was used to generate aspectrum out to ˜3500 nm. The same fiber is tested by first using anapproximately 2 m length of standard single-mode fused silica fiber(SMF). The output from the ˜2 m of SMF is then butt-coupled ormechanically spliced to the fluoride fiber of FIG. 12 b. In FIG. 13 athe long wavelength side of the spectrum is illustrated for differentlengths of the fluoride fiber at approximately 3 kW of peak power. For a˜1.8 m length of fluoride fiber, the spectrum 1310 reaches out toapproximately 3100 nm, meaning that this fiber length is too short forthe full spectral extent generation. On the other hand, at a ˜6 m lengthof fluoride fiber, the spectrum 1310 reaches beyond the spectral rangereached in ˜72 m of the same fiber 1320. Therefore, for the particularcircumstances of this experiment, the optimum length of the fluoridefiber is probably greater than 6 m, but shorter than 72 m.

The spectrum 1340 corresponding to using 5 m of the fluoride fiber fromFIG. 12 b after ˜2 m of SMF pre-stage fiber is illustrated in FIG. 13 b.The peak pump power in this case is ˜3 kW, and the spectrum 1340 is seento cover the range from ˜800 nm to ˜3600 nm. The short wavelength sideof the spectrum is collected using an optical spectrum analyzer, thelong wavelength side is collected using a grating spectrometer. The gapin the middle is due to filters use to insure that only the first orderlight of the grating is collected, and the higher orders from shorterwavelengths is blocked. Thus, with the appropriate pre-stage used tobreak up the pulses through MI, the fiber length required can reducefrom greater than 70 m down to less than 10 m.

The third fluoride fiber (characteristics in FIG. 12 c) has a cut-offwavelength of ˜2.75 microns, which would mean that the bend induced lossshould be well controlled to beyond ˜4.2 microns. To optimize the longwavelength edge from this fiber, the pre-stage fiber of SMF fused silicawas first optimized in length. For example, at a peak pump power of˜2.5-3 kW, it was found that approximately 1 m of SMF fiber with a zerodispersion wavelength around 1.3 microns gave the broadest spectrum. Inother words, at this power level the pre-stage SMF fiber helps to breakthe pulses up through the MI mechanism. Then, the pre-stage SMF fiber isbutt coupled or mechanically spliced to short lengths of the thirdfluoride fiber.

FIG. 14 a illustrates the long wavelength side of the SC spectrum fromapproximately 1 m of SMF fiber pre-stage followed by different lengthsbetween approximately 2 and 7 m of the third fluoride fiber (FIG. 12 c).For the 2 meter of fluoride fiber, the spectrum 1410 covers thewavelength range from the near-IR out to ˜4.2 microns. However, at thepump power used of ˜2.5 kW peak, the spectrum in this short lengthstarts to drop off at around 3 microns, suggesting that the fiber lengthmay be too short for the full spectral generation at this power level.When a ˜4.5 meter length of the same fiber is used, the spectrum 1430reaches out to ˜4.4 to ˜4.5 microns. When the length is furtherincreased to ˜7 m, the edge moves out slightly to approximately ˜4.5 to˜4.6 microns, but also the spectrum 1440 becomes more square-like (i.e.,higher spectral density further out in wavelength). Thus, for theparticular pre-stage SMF fiber and the pump power level, the optimallength for the third fluoride fiber would appear to be 4.5 meters orlonger.

FIG. 14 b illustrates the experimentally obtained power evolution of thespectrum from ˜2 m of the third fluoride fiber following anapproximately 1 m length of SMF fiber pre-stage. As the power increases,the spectrum 1450, 1460, 1470, 1480 is observed to increase in spectraldensity and also shift out to slightly longer wavelengths. As the plotshows, the spectrum is fairly well evolved by ˜2 kW of peak pump power,in this particular example.

The complete calibrated spectrum 1490 from ˜7 m of the third fluoridefiber following ˜1 m of SMF pre-stage is shown in FIG. 14 c. The longwavelength edge of the spectrum extends out beyond ˜4600 nm, and theshort wavelength edge of the spectrum extends beyond ˜800 nm. Thecomplete spectrum 1490 is obtained by connecting the spectrum from anoptical spectrum analyzer below ˜1750 nm with the longer wavelength datafrom the spectrometer followed by a cooled InSb detector. The data fromthe OSA is calibrated to obtain the spectral density in dBm/nm. Thenarrow peak near 1553 nm corresponds to the residual pump, and the peaknear 980 nm is the residual forward pump from the power EDFA stage.Furthermore, the bump near 980 nm corresponds to the ASE from the EDFAin the vicinity of the pump. The peak power from the pump isapproximately ˜4 kW, and the overall spectrum is seen to be quitesmooth. The fiber output from the SC fiber yields an average power of˜20 mW for this particular experiment. From the spectrum and themeasured output power, the conversion efficiency of the pump light tothe SC spectrum is approximately 50% or better.

One significant feature of the SC can be a high spectral density over awide wavelength range. For example, the spectrum 1490 in FIG. 14 c showsthat over a large part of the spectrum the average spectral density isbetween −25 and −18 dBm/nm (note that 1 dBm=1 mW). However, this is theaverage spectral density for a very low duty cycle pulse. For instance,with the ˜2 nsec pulses and 5 kHz repetition rate used in theseexperiments, the duty cycle is 1:100,000. Therefore, during the timethat the pulses are on, the actual peak spectral density is more like+25 to +32 dBm/nm. Thus, for a 10 nm bandwidth that might be used inspectral fingerprinting, the peak power is greater than 3 W. For a 100nm bandwidth that may be seen by one of the detectors in a heat sinkingmissile (e.g., as in typical in infrared counter measures), the peakpower is greater than 30 W. The pulsed mode used in the currentexperiments can be useful for lock-in or phase locked techniques thatuse detection systems such as box-car averagers, such as might be usedin spectral fingerprinting. In other words, to avoid collecting noiseduring the off-state of the light, the detection system canadvantageously only measure or record data during the on-state of theMIRFIL. In comparison to a broadband lamp, the average spectral densityin the SC is about 3×10³ brighter than a lamp and the peak spectraldensity in the SC is about 3×10⁸ brighter than a lamp. Thus, such abroadband mid-IR source can enable white light interferometrymeasurements with very high sensitivity.

Another feature of the SC 1490 is the remarkably smooth spectrum over awide spectral range. Because of the relatively stable pump laser inputto the SC fiber, it is believed that shot-to-shot the spectrum is thesame. In fact, this is a valuable attribute for spectroscopy. However,during the pump pulse, the less than 2 nsec pulse probably has a rangeof intensities. The different values of the intensity may in turn beresponsible for different parts of the spectrum. As a consequence ofaveraging over all the values of the intensities, the resulting spectrummay be quite smooth. This hypothesis also suggests a method of tailoringor adjusting the spectral shape of the SC. One way would be to usewavelength filters, such as gain equalizers or dynamic gain equalizers.However, another technique could be to modulate the time domain of thepump pulse, and then this temporal modulation could translate on to thespectrum as different parts of the pulse contribute to different partsof the spectrum.

There can be a number of techniques used to expand the long wavelengthedge of the SC generation in optical fibers. In one embodiment, thecomposition of the fluoride glass can be changed to permit transmissionout to longer wavelengths. The fibers described thus far are zirconiumfluoride glass, with a exemplary composition for the ZBLAN of (mole %):ZrF₄ (57), BaF₂ (34), LaF₃ (5), AlF₃ (4). For the ZBLAN or moregenerally the zirconium fluoride fibers, the transmission edge of 1 dB/mat 4.3 microns is fairly common, and it the IR edge does not shift veryeasily. On the other hand, fluoride glass fiber that does not containzirconium fluoride fiber or other short-IR-edged compounds may enabletransmission to longer wavelengths. By changing the composition, thelong wavelength edge can be found to extend beyond ˜5.4 microns.Therefore, if the SC generation were implemented in such a fiber, theedge of the SC might be expected to reach beyond 5 microns. For longwavelength performance, the cut-off wavelength for the fiber shouldprobably be beyond 2 microns, preferably beyond 2.5 or 3 microns, tocontrol the bend induced loss at the longer wavelengths. The core sizecan also be advantageously relatively small (e.g., less than a corediameter of 12 microns, more preferably less than 10 microns) to reducethe power requirements for the SC generation. However, larger core sizesmay also be used to increase the overall output power from the SCspectrum.

Other embodiments of fluoride fiber can also be used to extend the longwavelength edge or to optimize the shape of the SC spectrum. In oneembodiment, the pump wavelength could be made closer to the zerodispersion wavelength of the fiber, or a cascaded Raman shifted order ofthe pump could fall closer to the zero dispersion wavelength of thefiber. In a preferred embodiment, the pump or the shifted pumpwavelength would fall slightly to the long wavelength side of the zerodispersion wavelength. This would lead to MI with a broad gain spectralwidth. In another embodiment, a hybrid configuration of differentfluoride fibers could be used to effectively taper the core size of thechain, either downward or upward. In yet another embodiment thewavelength dependence of the core and cladding material can be selectedso that the numerical aperture (NA) increases with increasingwavelength. For a step-index fiber, the NA=sqrt (n₁ ²−n₂ ²), where n₁ isthe index of the core and n₂ is the index of the cladding. Therefore, ifthe difference between the two indices of refraction increases withincreasing wavelength, then the NA will increase. As the NA isincreased, the waveguide will be better guiding and the effect of bendinduced loss will be lowered.

As an alternative, fibers made from different materials can also be usedto increase the wavelength extent of the SC. Another option for mid-IRfibers are tellurite (TeO₂) glass fibers. Recently, there has beengrowing interest in the TeO₂-based glasses because of their strongnonlinear properties and capacity for doping with high concentrations ofrare-earth elements. Hence, these glasses can be appropriate for a widerange of devices including lasers, amplifiers, and mid-IR wavelengthconverters. Several preliminary studies have been reported in theliterature regarding the glass properties. For example, depending on thedoping details, the Raman gain coefficient can range from 30 timeslarger than fused silica to 45 to 95 times larger than fused silica. Inaddition, the Raman gain band in the TeO₂ glasses can be up to a factorof two wider in bandwidth than fused silica. Moreover, the damagethreshold for the TeO₂ glasses is measured to be approximately 15-20GW/cm², which is about a factor of two or three smaller than fusedsilica at 50 GW/cm². For the tellurite fibers the nonlinearity can bestrongly dependent on the material composition, and the zero dispersionwavelength can also vary with material composition. In addition, thetellurite fibers may transmit light at least out to 4 microns, and evenout to 5 microns in bulk glass. According to some reports, at 4 micronsthe theoretical background loss (i.e., material loss) can be somewhereabove 10 dB/m. The minimum loss in tellurite fiber would be around 3microns, and the value of the loss should be between 5-10 dB/m in thefiber at 3 microns. In yet other embodiments, materials made inwaveguides may be advantageous for mid-IR light generation. For example,if the pulse break-up first occurs in fused silica fiber, then thenonlinear spectral broadening for SC generation can occur in silicon orother semiconductor waveguides.

Given that only certain range of fiber parameters are available in thefluoride fibers and that step-index fiber can only provide limitedcontrol over the dispersion profile, an additional degree of freedom forthe mid-IR fibers may be helpful. The use of two pump wavelengths mayprovide this optimization option. With the two pump case, MI can occurwith either pump in the anomalous or normal dispersion regime. Thus,whereas for the single pump case MI phase matches when the pump is inthe anomalous dispersion regime, the addition of a second pump relaxesthis constraint. As an example, FIG. 15 illustrates the use to two pumpwavelengths falling in the anomalous dispersion regime in the ZBLANfluoride fiber. In particular, the zero dispersion wavelength frommaterial dispersion is at 1628 nm, and pumps at 1630 and 1635 nm areassumed. The pump at 1630 nm is assumed to be 3.5 kW peak power, and thepump at 1635 nm is varied at 0 1510, 1 kW 1520, 2 kW 1530 and 3.5 kW1540. As the second pump is increased, the gain bandwidth stretches from3.7 microns to 4.2 microns, 4.9 microns and approximately 6 microns.These two pump wavelengths can be implemented directly with EDFAamplification (using co-called L-band amplifiers), or they can begenerated near 1530 nm, and then one Raman wavelength shift can be usedto transfer the energy closer to 1630 nm. In another embodiment, anadditional degree of freedom can be obtained in fluoride fibers by usingmicrostructure fiber geometries, which are also often called photoniccrystal fibers.

Another aspect of the MIRFIL is that the average power can be increasedto >500 mW from the current ˜20 mW average power. For the higher powers,one change could be to use a higher power pump laser. Examples of higherpower pump lasers include solid-state lasers, diode-pumped laser systemsincluding solid state lasers, cladding pumped fiber amplifiers andlasers, and optical parametric oscillators or amplifiers. To improve theefficiency and power, longer wavelength (˜2 microns) and higher powersolid state lasers or cladding pumped fiber amplifiers or lasers canalso be used. For instance, holmium or thulium lasers provide light near2 microns in wavelength. As the powers are increased, another change canbe to use larger core size fibers, so that the intensities can remainbelow the damage threshold while the overall output power can beincreased. For example, different core sizes of fluoride fibers arealready commercially available. In addition, the HiNL fused silicafibers could possibly be pulled to larger sizes, although care will beneeded not to change the zero dispersion wavelength in these fibers.

Although a number of embodiments of using fluoride fibers to generate SCinto the mid-IR are described, other configurations and fiber types canalso be used to alter the shape of the SC spectrum or to extend thewavelength range of the SC generation.

Generalization of SC or Wavelength Conversion and Using SemiconductorWaveguides

The results in the fused silica and fluoride fibers suggest a moregeneral model of optimizing SC generation or wavelength conversion(further discussed in a few sections below). One example of thegeneralized model is illustrated in FIG. 16. The light originates from apump laser 1610, which can a laser diode followed by EDFA's, claddingpumped fiber amplifiers or lasers, diode-pumped solid state lasers,diode-pumped fiber lasers, or any number of light sources in the near-IRwavelength range. It may be desirable to include a wavelength shifter1640 (dotted line boxes correspond to different optional elements in theoptimized set-up). As an illustration, the wavelength shifter 1640 mightbe a Raman wavelength shifter, a cascaded Raman oscillator, an opticalparametric oscillator or an optical parametric amplifier. In addition,it may advantageous to introduce light from a seed laser 1650, which canbe a laser diode, a tunable laser diode, a fiber laser, a solid statelaser, or another super-continuum source. In the case of the experimentsto date, the seed light may be arising from the ASE from the opticalamplifiers. However, if the optical amplifier is not used, then it maybe advantageous to introduce a seed laser light to lower the thresholdor control the wavelength of the modulational instability in the nextstage.

The first stage may be used to cause break-up 1620 of the CW or quasi-CWlight into pulses or solitons through the MI effect. The first stage1620 can advantageously be implemented in optical fibers, and for asingle pump wavelength the MI phase matches in the anomalous groupvelocity dispersion regime. If the pulses are nano-second (i.e., longerthan approximately 100 psec, or even longer than about 30 psec) orquasi-CW light, there may be enough intensity modulation to causecollisions between different soliton pulses. Otherwise, in a preferredembodiment an intensity modulator can be used to create a distributionof intensities, which in turn can lead to a collision between solitonpulses. The intensity modulation may also help to create a smoothspectrum, due to the distribution of pump intensities.

In some cases, it may be further advantageous to have a mid-stage 1660after the MI-initiated pulse break-up stage. This mid-stage, forexample, can have a pulse sharpener 1660, which helps to compress thesoliton pulses and/or create more modulation sidebands in the frequencydomain. Examples of the mid-stage include optical fibers, dispersiondecreasing fibers, tapered fibers, grating compressors, or otherexamples of pulse compressors, whether they are implemented in opticalfibers or bulk optics. This mid-stage can additionally help byincreasing the peak intensity of the pulses. As such, the mid-stage canalso include optical amplifiers.

The second stage can then include a nonlinear element for SC generation1630 or wavelength conversion 1670. The non-linear element can help togenerate SC or new wavelengths based on four-wave mixing processes. ForSC generation, the nonlinearity in the second stage can give rise tospectral broadening through self-phase or cross-phase modulation.Although the nonlinear properties of this second stage is one of theimportant parameters, it may also be desirable to have some dispersionto cause pulse walk-off or pulse motion. Such pulse motion may help tosmoothen the spectrum or create even higher peak intensities. It mayalso be advantageous for the second stage to be at least partiallytransparent over the wavelength of interest. For example, for mid-IRconversion, it may be advantageous for the second stage to betransparent over much of the mid-IR wavelength range. Examples of thesecond stage include different optical fibers, including HiNL, ZBLAN,fluoride, tellurite, chalcogenide, or even semiconductor doped glassesor waveguides.

Although most of the experiments presented have used mid-IR fibers orfused silica fibers, in the more generalized model other elements suchas semiconductor waveguides or nonlinear crystal material could be usedin the second stage. As one particular example, a silicon waveguidecould be used as the second stage. The nonlinearity in silicon is aboutfour-orders-of-magnitude higher than in silica fiber. The band gap insilicon is around 1.1 microns, so silicon is transparent (at least in alinear sense) for wavelengths longer than 1.1 microns and throughout themid-IR wavelength range. Therefore, it is advantageous to have a pumpwavelength below the band gap of silicon. However, for a pump wavelengthbetween approximately 1.1 and 2.2 microns, the pump will experiencetwo-photon absorption (TPA). In turn, the carriers generated through TPAcan induce free-carrier absorption.

One method to overcome the TPA-induced free carrier absorption is toembed the silicon waveguide in a P-I-N diode configuration, particularlywith the PIN diode reverse biased. As an illustration, the waveguide mayfall in the I (intrinsic) region, and the electric field from thereverse biased diode can help to quickly sweep out the electrons andholes created by the TPA effect. Although this technique reduces thefree carrier absorption, it does not prevent the TPA effect.Furthermore, the silicon waveguide in a PIN diode can be enhanced in anumber of ways For instance, the length of the waveguide can be extendedby using multiple zigzags, such as in a S-configuration. Moreover, thepump light can be multiple passed by placing coatings on thesemiconductor wafer or mirrors around the wafer. In a preferredembodiment, one side of the chip may be coated for high reflectivity,while the other side can be anti-reflection coated or dichroic coated.Another advantageous configuration can modulate the applied voltage tothe PIN diode to control the loss in the waveguide. As an example, thismodulation could control the long wavelength edge of the SC spectrum orcould be used to put codes onto the SC spectrum.

The silicon PIN waveguide is just one example of the nonlinear element1630 or 1670 that could be used for SC generation. There are many othersemiconductor or other materials that could alternatively be used. Forinstance, a waveguide can be made in a wide-gap semiconductor, where theband gap is at shorter wavelength than the TPA edge. This would avoidthe TPA problem, thereby removing the necessity of using a PIN forcarrier sweep-out. Alternately, a more atomic-like material can be used,such as quantum dots, so the material does not have a conduction bandand the associated TPA problems. Moreover, other nonlinear crystalscould be used, such as lithium niobate or periodically-poled lithiumniobate. Furthermore, different fiber configurations could be used. Forexample, a hollow core fiber or capillary could be used that is filledwith a nonlinear liquid, such as CS₂. Other fiber types could also beused, such as tellurites, chalcogenides, or photonic crystal fibers.

Cascaded Raman Wavelength Shifting in Chalcogenide Fibers

Chalcogenide fibers represent another alternative of fiber types formid-IR light generation. Examples of chalcogenide fibers include sulfide(typically transmitting out to approximately 6 microns), selenide(typically transmitting out to approximately 9 microns) and telluride(typically transmitting out to 11 microns). Technical feasibility hasbeen demonstrated for cascaded Raman wavelength shifting in chalcogenidefibers. In a particular embodiment, samples of arsenic-tri-sulfidefibers were obtained. The testing started with a 20 m length of fibernumber WS-884, which has a slight selenide doping, a core size ofapproximately 6.5 microns, and a numerical aperture of ˜0.22. Forexample, FIG. 17 shows the spectral output from about 12 m of the WS#884fiber for different input peak powers. The second cascaded Raman ordercan be observed at ˜200 W peak power input to the chalcogenide fiber.Also, this second cascade order can be repeatable, and it grows to anoticeable strength by ˜235 W peak input power 1710 (this is powerincident on the fiber, not necessarily the fiber coupled into thefiber).

To generate and measure the spectrum beyond the second Raman cascadeorder 1770, the light from the mid-IR fiber can preferably be sent to anoptical spectrometer that is optimized for the near to mid-IR. Inparticular, a 0.3 m spectrometer is used that has a grating with 300grooves/mm. The numerical aperture for the fiber output is optimized tocouple into the spectrometer using lenses that are transmitting in themid-IR, such as calcium fluoride lenses. The detector used is a modifiedInGaAs detector, which has high sensitivity out to 2.6 microns. Tominimized the effect of the water absorption line around 1.9 microns, adry nitrogen as is used to purge the interior of the spectrometer.

FIG. 17 b illustrates the spectrum 1740 at the output of fiber WS#884measured using the optical spectrometer. With the extended range of thespectrometer and the nitrogen purge, the third Raman cascade order 1780can be observed. The pump power incident on the sulfide fiber is nowraised to approximately 350 W. As the pump power is raised, the thirdorder 1780 cascaded Raman wavelength shift grows. It should be notedthat the actual third order shift is probably higher in magnitude, sincethe path from the fiber to the spectrometer is not purged and there maystill be residual moisture in the spectrometer chamber. Further ordersof cascaded Raman wavelength shifting may be limited by damage at theinput to the fiber as the pump power is raised.

The results from the chalcogenide fibers could be improved using anumber of techniques. Different fiber sizes will be tested to see if thefiber core is more uniform or continuous in the larger core size fibers.Gallium on the two ends of fiber can be used to test for the guidingproperties of the lowest order mode in different fiber lengths. The endsof the sulfide fiber may also be encapsulated to remove heat and,thereby, to increase the damage threshold.

As another alternative, selenide fibers could be used, which areinteresting because they should have an order of magnitude larger Ramangain coefficient compared to the fibers tested. One question is thevalue of the damage threshold power for the selenide fibers. If thedamage threshold is the same in the selenide fibers as the sulfidefibers that have been tested, then a significant improvement in CRWSmight be expected. However, the index-of-refraction variation withtemperature ∂n/∂T can be positive in the chalcogenide fibers, and thevalue can be an order of magnitude larger in the selenide fiberscompared to sulfide fibers. Therefore, one concern may be thatcatastrophic self-focusing might occur in the selenide fibers due tothermal effects from light absorption. In addition, the selenide fibershave a band gap of ˜750 nm, which is closer to the pump wavelength thanthe sulfide fibers (band gap around ˜520 nm). Thus, a second concernarises from photo-darkening effects arising from two-photon absorption.To overcome photo-darkening concerns, it might be worth trying a hybridapproach, where light is first shifted in fused silica out to ˜2-2.8microns, and then the light is coupled into the chalcogenide fibers forfurther shifting. An alternative approach will be to pump thechalcogenide fibers with thulium lasers (either fiber based or solidstate lasers), so the shifting starts from around 2 microns. Althoughparticular schemes are described for CRWS in chalcogenide fibers, amyriad of other techniques and materials can be used for generatinglight using CRWS into the mid-infrared.

Wavelength Conversion Based on Four-Wave Mixing

There are applications, such as spectral fingerprinting, where SCgeneration can be very valuable. Also, SC could benefit infraredcounter-measures (IRCM), because it becomes virtually undefeatablebecause the broad spectrum mimics the black body radiation from hotmetal objects. However, there are many cases where only a narrow band offrequencies in the mid-IR may be desired. For example, laser ablationtypically only uses a band of frequencies, and IRCM traditionally usesthree frequency windows in the mid-IR. For these cases where only a fewmid-IR wavelengths are required, SC can be inefficient, since the energymay be spread over a wide spectral range. Wavelength conversion of thepump wavelength to a set of frequencies in the mid-IR would besignificantly more efficient.

Because of the similarity of the experimental set-up and the sameunderlying physics at work, one question is when does SC generationoccur and when does wavelength conversion occur. The MI process can beused to convert the CW or quasi-CW (e.g., nanosecond pulses) to shortpulses required for many of the nonlinear phenomena, thereby reducing oreliminating the need for modelocked lasers. Also, for the single pumpwavelength case MI phase matches in the anomalous group-velocitydispersion regime. Therefore, the first step for either SC or wavelengthconversion can be to propagate the light in a length of anomalousdispersion fiber (i.e., soliton regime of the fiber). The maindifference in outcome may depend on how long the pulses are permitted topropagate in the soliton regime of the fiber.

To distinguish SC generation from the wavelength conversion processes,it is worth first examining the onset of the SC generation process. Asan example, FIGS. 5 and 6 show simulations of the break-up of quasi-CWpulses through MI and then the onset of SC generation. FIG. 5 shows theinitial break-up of the quasi-CW pulse into solitons, and then the Ramaneffect shift in energy to the longer wavelength side (left side is timeand right side is frequency domain). Thus, the broad quasi-CW input isbroken into a train of solitons.

FIG. 6 shows one example of the time-domain collision process furtherdown the fiber, as the onset of SC generation can be seen on thecomputer. The red-shifted pulses travel through the blue-shifted pulsesbecause of the anomalous dispersion, and then the red-shifted pulses robenergy from the blue-shifted pulses through the Raman effect. Thiscomplicated collision process may give rise to narrow, high peak powerpulses, which can lead to SC generation. The generation of the largesuper-pulses in FIG. 6 may be advantageous for achieving the extremelyhigh intensities and the run-away effect that give rise to SCgeneration. Note that the collision of the pulses occurs becauseself-phase modulation leads to the initial red-shifting of the leadingedge of the pulse (i.e., the part of the pulse that occurs earlier intime). Then in the anomalous dispersion regime the red-shifted pulsestravel slower, causing the pulses in the leading edge of the pulses topass through the pulses in the trailing edge of the pulse (FIG. 6). Asthe red-shifted pulses travel through the other soliton pulses, throughthe Raman process the red-shifted pulses grow in energy and furthernarrow.

In order to observe wavelength conversion through four-wave-mixing(4WM), the MI break-up of the pulses as seen in FIG. 5 can beadvantageous, but the super-pulse creation process of FIG. 6 that leadsto SC generation should preferably be avoided. As a specific example,the purpose of the ˜0.5 m length of standard SMF fiber (fiber that canbe in the soliton regime or anomalous dispersion regime) in theexperiments is to convert the ˜1.8 nsec pulses from the laser into theshort soliton pulses. This length may be intentionally kept short toavoid the collision phenomena of FIG. 6.

Depending on the wavelength conversion mechanism, different strategiescan be used to avoid the collision and super-pulse creation of FIG. 6 inthe second stage of fiber. As an example, to observe wavelengthconversion through 4WM, the second stage fiber is selected to operate inthe normal dispersion regime. Since the red-shifted pulses travel fasterthan the blue-shifted pulses in the normal dispersion regime, thecollision and super-pulse formation of FIG. 6 are avoided. For singlepump wavelength seeded MI, anomalous dispersion is required for phasematching. Therefore, by using normal dispersion in the second stage, therun-away effect of MI can be avoided, and 4WM can phase match to providethe wavelength conversion.

Four-wave mixing is a four-photon process where two pump photons combineto produce a Stokes wavelength (longer wavelength) and an anti-Stokeswavelength. One aspect of 4WM is that phase matching is required betweenthe four waves. For instance, the wave vector mismatch is given by

${{\Delta\; k} - {2k_{p}} - k_{s} - k_{a}} = {2\frac{n_{{p{(1)}}p}}{c}\frac{n_{{s{(1)}}s}}{c}\frac{n_{{a{(1)}}a}}{c}}$

and the conversion efficiency is given by

$\eta_{4{WM}} = {( {\gamma\;{PL}} )^{2}\begin{matrix}{\sin^{2}\begin{pmatrix}{\Delta\;{kL}} \\2\end{pmatrix}} \\\begin{pmatrix}{\Delta\;{kL}} \\2\end{pmatrix}^{2}\end{matrix}}$

Normally, high efficiency for the 4WM process can be obtained near thezero dispersion wavelength. However, a new regime of phase matching canbe advantageously used that enables mid-IR light generation, since thisnew regime is distant from the zero dispersion wavelength. As oneexample, the 4WM wave vector mismatch 1800 is calculated and plotted inFIG. 18. Assuming a zero dispersion wavelength near 1.7 microns for thefluoride fiber, the wave vector mismatch can be small close to zerodispersion, such as for wavelengths around 1.6 to 1.8 microns. However,there turns out to be another zero crossing in this case around 1.02microns. The 4WM efficiency turns out to be large above 1.5 microns, butalso large in the vicinity of 1.02 microns. Although this second windowgenerally is found to be narrower bandwidth, it can give rise towavelength conversion into the mid-IR. For instance, for a pumpwavelength of 1553 nm and the anti-Stokes wavelength of 1020 nm, theStokes light generated would be in the vicinity of 3.36 microns.

As the zero dispersion wavelength and the dispersion profile of thefiber is changed, the position for this mid-IR light wavelengthconversion can change. For instance, the following table shows differentexamples of the calculated and measured 4WM peak for different fibersmeasured.

Theoretical Experimental γ₀ (um) Peak (um) Peak (um) 1.56  1.37 1.40 (→1.75) 1.57* 1.25 1.23 (→ 2.11) 1.61* 1.12 1.17 (→ 2.31) 1.70# 1.01 1.02(→ 3.25)

Experimental confirmation can also seen of this new regime of phasematching for 4WM in different fused silica and fluoride fibers. As aparticular example, the experimental set-up for testing 4WM wavelengthconversion is illustrated in FIG. 19. The pump 1910 is similar to thatof the SC experiments. However, at the output of the power amplifier isa WDM 1930 or power dividing coupler to inject a seed wavelength 1920,and this is followed exemplary by an approximately 0.5 meter length ofSMF fiber 1940 (this fiber, in many cases, can just be the fiberpigtails of the coupler). This pre-stage fiber may serve to break up thepulses through MI, but the length is maintained short enough to attemptto avoid SC generation. Then, the output of the SMF pre-stage fiber iscoupled to various fibers 1950, which are preferably in the normaldispersion regime for the pump wavelength. In a preferred embodiment, aseed laser diode would be placed at the anti-Stokes wavelength, and theStokes wavelength would be generated through the 4WM process.

The data above suggests a procedure for wavelength conversion of lightinto the mid-IR wavelength range, particularly when there is a targetwavelength desired for a particular application. First, the dispersionof the fiber can be tailed to phase match at a target wavelength. Thefiber dispersion can be tailored by changing the zero dispersionwavelength, adjusting the dispersion slope, or perhaps by using moreexotic fibers such as micro-structure fibers that can have more than onezero dispersion wavelength. Then, if the pump is for example within thetelecommunications band, then tune the wavelength of the pump laser toobtain the correct target wavelength. The pump laser could be a tunablelaser, or the pump laser could be laser diodes of different wavelengths,for example laser diodes that are on the ITU wavelength grid. With theappropriate adjustment of the phase matching condition, then introduce aseed laser at the anti-Stokes wavelength. Since the anti-Stokeswavelength falls in the near-IR wavelength range, one example of a seedlaser would be laser diodes. With the introduction of the anti-Stokeswavelength, mid-IR light on the Stokes side should result, so long asthe fiber can transmit light at the particular mid-IR wavelength. Thus,as an example light out to ˜2.7 microns might be generated in fusedsilica fiber, light out to ˜4.4 micron might be generated in ZBLANfluoride fiber, and light out to ˜5.5 microns might be generated in theextended band fluoride fiber. In a preferred embodiment, a fused silicafiber pre-stage can be used to generate pulses through MI, and then thewavelength conversion would be in fiber where the pump wavelength fallsin the normal dispersion regime. Although one particular method ofwavelength converting light into the mid-infrared regime is suggested,numerous other techniques can be used within the scope of the presentdisclosure.

Applications of MIRFIL Sources

Several differentiators for the MIRFIL fiber-based sources include:Maturity of underlying technology; for SC, emulate black body radiationor attractive source for spectral fingerprinting or last mile solutionsin telecommunications; for wavelength conversion, simple tuning overwide wavelength range Excellent beam quality (M²<1.4, as an example);advantages of fibers, such as compact, robust, lightweight, and nomoving parts; potential room temperature operation with flexiblerepetition rate from CW to MHz or higher; and power scalable to ˜10 W ormore by using larger core size fibers and higher pump powers.

On this last point, the scalability of the power by pumping with acladding pumped fiber laser can be quite attractive. As an example, inthe last several years the CW power from cladding pumped fiber lasershas increased from 10's of Watts to a time-average power of 15 kW in2005. Moreover, pumping with a cladding pumped fiber laser could enablean all-fiber integrated MIRFIL. The SC generation or wavelengthconversion fibers (whether one, two or more stages) could be coupled tothis pump unit using either fusion splicing, mechanical splicing, orfree space or bulk optical coupling. Then, the resulting unit could bean all-fiber, high power MIRFIL. As mentioned before, cladding pumpedfiber lasers can operate at exemplary wavelengths near 1 microns, 1.55microns or 2 microns, depending on the dopants in the fiber. Although aparticular monolithically integrated MIRFIL is illustrated, many otherconfigurations and pumping techniques can be used within the scope ofthe present disclosure.

The MIRFIL may be used for applications where light in the mid-IRwavelength range (exemplary 2 to 5 microns) is advantageous. Forexample, the mid-IR is known as the spectral fingerprint region, becausemany chemicals have their rotational and vibrational resonances at leastin part in the mid-IR wavelength range. Also, the mid-IR can beimportant for heat sensing, since black body radiation from “hotobjects,” such as plumes or hot metal, falls at least in part in themid-IR. Moreover, for applications in the life sciences, laser ablationnear 3.6 or 6.45 microns could be advantageous, since the protein andamide group absorption can exceed the water absorption. Also, mid-IRlight near the peak of the water absorption could lead tohigh-resolution photo-acoustic imaging, which can be important forapplications such as laser keratectomy. These are exemplary applicationsof mid-infrared light sources, but many other applications fall withinthe scope of the present disclosure.

The early adaptors of the MIRFIL laser technology may be in militaryrelated markets for chemical sensing and infrared counter-measures(IRCM). However, there are also commercial markets for the same kind ofMIRFIL laser units. For example, a similar laser that is used forchemical sensing can be used in the commercial sector for industrialchemical plant control, advanced semiconductor processing, combustionmonitoring and bio-medical diagnostics. Similarly, a similar laser thatis used for IRCM can be used in the commercial sector for bio-medicallaser ablation.

The first application to use the MIRFIL may be chemical sensing systemsproducts. In particular, the wavelengths of IR absorption bands arecharacteristic of specific types of chemical bonds and every moleculehas a unique IR spectrum (fingerprint). IR spectroscopy finds itsgreatest utility for identification of organic and organo-metallicmolecules. There are three IR spectroscopy technologies employed inpoint detectors: Fourier transform IR (FTIR) spectroscopy,photo-acoustic infrared spectroscopy, and filter based IR spectroscopy.

The SC broadband source could be particularly useful for spectralfingerprinting. In several chemical sensing detection systems, a narrowline width, tunable laser may be used to perform spectralfingerprinting. Instead of this approach, the SC based spectralfingerprinting can be much more like white light spectroscopy. In otherwords, the SC may permit simultaneous monitoring over a wide spectralrange. In one embodiment, the spectra at several wavelengths can be usedto advantageously identify a chemical species. In another embodiment forabsorption or reflection spectroscopy, several wavelengths of theabsorption or reflection can be measured either simultaneously or insome time sequential fashion. Then, the relative magnitudes at differentwavelengths or a particular spectral pattern of absorption or reflectioncan be pattern matched to identify the chemical of interest. Such atechnique has the potential of having high selectivity, since themonitoring can be accomplished over a wider spectral range and since thespectral pattern matching can compare a number of features.

An exemplary system 2000 for performing spectral fingerprinting or usingthe light source is illustrated in FIG. 20. The chemical sensing systemscan include a light source 2010, such as the MIRFIL light source,filters, and a lens system to transmit through a sample or sample volume2030. There may be a reference path 2020 for calibrating the system. Inanother embodiment, the reference path may substantially coincide withthe sample path 2030, but the two can be time multiplexed—i.e., thereference signal may be at a different time than the sample signal. Thesample path 2030 can collect the light in transmission or reflection,depending on whether the detector is integrated with the light source orin a different location. The light detection system 2040 collects atleast a fraction of the light, and data collection and analysis computersoftware 2050 may be coupled to the detector and receiver (i.e.,electronics behind the detector). As an example, the light detectionsystem 2040 can include a grating and a linear array of mid-IRsemiconductor detectors or multi-spectral detectors. Alternately, thedetection system 2040 can be a moving grating and slit or a MEMS-basedgrating followed by a detector. In a preferred embodiment, when aparticular wavelength range is being detected, narrow-band detectors orfilters followed by detectors could be used to select only thewavelength of interest and reject the noise and signals at otherwavelengths. One advantage of the system 2000 of FIG. 20 is that it maylead to non-contact, remote detection of chemical species. In such asystem, some of the important issues are the sensitivity and selectivityor interference between the signatures of different chemicals.

Systems such as 2000 can be used for chemical sensing for militaryapplications as well as industrial plant monitoring systems. Forexample, chemical sensing can be used to detect chemical warfare agents,which are chemical substances that are intended for use in warfare orterrorist activities to harm people through their physiological effects.The most common chemical agents include nerve agents, blister agents andarsenical vesicants. Moreover, chemical sensing can be used for weaponsdetection, since residue from gun power can be sensed using remote ornon-contact optical spectroscopy techniques. In addition, toxicindustrial materials are chemicals other than chemical warfare agentsthat have harmful effects on humans. These are used in a variety ofsettings such as manufacturing facilities, maintenance areas, andgeneral storage areas.

In one embodiment, the spectral fingerprinting system can be used forfirearms detection. For example, firearms detection can be implementedby searching for the composition of gun powder. One chief ingredient insmokeless gun powder is nitro-cellulose, which has clear spectralfeatures centered around 2.86 microns and 3.45 microns. Although thereare also lines at 6 microns on beyond, many chemicals have a lot oflines in that wavelength range, so it may be difficult to separate onechemical from another. Beyond nitro-cellulose, there are also a numberof additives in smokeless gun powder, an example of which is provided inthe table below.

Component Function Typical chemicals Plasticizer Organic materials addedto aid Dibutyl Phthalate fabrication of propellants and explosivemixtures Primer Initiate the propellant in Lead azide ammunition Leadstyphnate Tetracene Barium nitrate Strontium nitrate Stabilizers Organicmaterials that retard Diphenylamine decomposition of other EthylCentralite constituents during storage Propellant Organic materials thatundergo Nitroglycerin rapid combusion

Diphenyl amine, which is used extensively as a stabilizer, shows clearspectral signatures centered around 2.94, 3.33 and 3.85 microns. Dibutylphthalate, which is used as a plasticizer, shows an absorption peakaround 3.4 to 3.55 microns. Lead azide, which is used as a primer, has apeak absorption around 4.8 microns. Other examples of primers includetetracene (broad absorption between approximately 2.8 and 4 microns),barium nitrate (absorption peaks near 2.94 and 4.2 microns), andstrontium nitrate (absorption peaks around 2.94 and 4.15 microns). Thus,many of the components of smokeless gun powder have signatures in themid-IR between 2 to 5 microns.

In another embodiment, the spectral fingerprinting system can be usedfor IED (improvised explosives detection) or weapons detection. Many ofthe explosives have modified benzene rings, and the benzene rings have aresonance around 3.2 microns. Although there are a lot of absorptionlines from 6-10 microns and in the terahertz region, it may be difficultto sort out one chemical from another (i.e., there may be too muchinterference, leading to poor selectivity). Cleaner, more discrete,signatures are seen in the mid-IR, so although the level of absorptionmay not be as great, the selectivity may be better. Examples ofexplosives and their approximate mid-IR lines include the following:

PETN (pentaerythritol tetranitrate) 2.67, 3.57, 4.25 microns RDX(cyclotrimethylenetrinitramine) 2.9, 3.23 microns TNT 2.9, 3.23 micronsTetryl (2,4,6-Trinitrophenylmethylnitroamine) 2.9, 3.23 microns HMX 2.9,3.3 microns Ammonium nitrate broad centered 3.23, narrow 4.1 microns

There are other applications in chemical sensing for the spectralfingerprinting system as well. For example, the system can be used fordrug detection or chemical weapons agent detection. As an illustration,drugs such as cocaine, methamphetamine, MDMA (ecstasy) and heroin havedistinct optical spectral signatures in the wavelength range from 2-5microns. In one embodiment, the use of a broadband source covering alarge fraction of the mid-IR between 2 to 5 microns can be used toadvantageously detect various drugs. Moreover, many of the chemicalweapons agents, such as sarin, cyclosarin, soman, tabun, sulfur mustard,nitrogen mustard, VX and lewisite, have absorption features in the 3 to4 micron window, particularly centered around 3.3 microns. Thus,non-contact, remote detection of drugs, weapons, firearms, and chemicalagents could advantageously be implemented with a spectralfingerprinting system utilizing the SC source.

Beyond chemical sensing, another application for the high power versionof the MIRFIL in military and homeland security might be in IRCM,particularly for the commercial air fleet. For instance, much of theblack body radiation falls in the wavelength range covered by the SCsources described between ˜1 microns and ˜4.5 microns. In oneembodiment, the SC spectrum could be carved or shaped using spectralfilters to resemble the spectrum for hot metal or plume.

Other chemical sensing applications for the SC source or wavelengthconversion source include semiconductor process control, combustionmonitoring and industrial chemical. For example, the chemicals in thesemiconductor growth chamber can be monitored to provide a real timefeedback signal to an advanced process control engine. By using the SC,a number of chemical species can even be monitored simultaneously.Examples of chemicals that are relevant for semiconductor processinginclude monitoring HCl and HBr for plasma etching or monitoring CxFy forgate etching. Alternatively, the chemicals in combustion chamber can bemonitored using spectral fingerprinting. Most applications relevant togas dynamic and combustion flows are based on absorption bylow-molecular weight molecules with well resolved transitions—such asO₂, H₂O, CO, CO₂, NO, NO₂, OH, NH₃, HF, H₂S, CH₄, as particularexamples. Because of current limitations arising from a lack ofconvenient mid-infrared sources, the absorption measurements today forchemical sensing may be performed on overtone and combinationalvibrational absorption bands, which typically fall in the near IR wherelaser diodes are available. However, typical line strengths of thesetransitions are two or three orders-of-magnitude below the fundamentalvibrational transitions in the mid-IR. Therefore, by using SC in themid-IR wavelength range, a much stronger signal can potentially beobtained by operating at the fundamental wavelength of the transitions.

Another application for the MIRFIL based on SC generation or wavelengthconversion is in bio-medical ablation or imaging. As an example, theprotein absorption dominates over water absorption between ˜3.6 micronsand ˜4 microns and again near 6.1 microns and 6.45 microns. By usinglaser ablation in one of these windows, the protein can be denatured(for example, by relying on the amino acid absorption) before boilingthe water, thereby resulting in less collateral damage. One example ofthe value of avoiding the collateral damage could be in cosmeticsurgery. For instance, cosmetic surgery is often used to remove wrinklesor unwanted skin or tissue, but discoloration or scars from heatingmight be undesirable. By denaturing the protein with minimum collateraldamage, the unwanted skin or tissue or wrinkles could be removed withoutscaring or skin discoloration. To achieve the wavelength range ofinterest, SC generation or wavelength conversion could be used based on4WM. This is one example of a mid-infrared light source for biomedicalapplications, but many other configurations can be used within the scopeof the present disclosure.

The above example uses mid-IR light in applications at wavelengths wherethe protein absorption exceeds the water absorption. However, there areseveral instances where the optimal use of the mid-IR light can be atwavelengths where the water absorption dominates. In a particularembodiment, the mid-IR light can be used at a wavelength of strong waterabsorption, such as close to 2.9-3.1 microns, so that a short ultrasonicor acoustic wave can be launched for high-resolution ultrasound imaging.The wavelength of strong water can be selected to minimize theabsorption length of the mid-IR light in the water. In a preferredembodiment, the pulse width of the mid-IR light is under 100 nsec, under10 nsec or under 2 nsec. These wavelengths and pulse widths areexemplary, but many other ranges of values can be used.

For the short pulses and absorption lengths, the resulting wave thenacts as an acoustic impulse. As an example, one particular embodimentwhere the acoustic impulses can be beneficial is in precise corneathickness measurements (pachymetry) made during planning for laserkeratectomy. Precise thickness measurements can be obtained withhigh-frequency ultrasound. The use of optical pulses at wavelengths ofhigh water absorption to create the acoustic pulses lends itself to anon-contact procedure for ultrasonic measurements. On benefit of usingan all-optical method to generate the acoustic wave can be that itenables simple integration with laser ablation systems. Thus,measurements and laser ablation can be done in one proceduresequentially without need for moving instruments or patients. Moregenerally, laser-induced ultrasonics operating near the water absorptionlines can be used to map out many different materials and biologicalsystems. For these types of application, it could be more advantageousto wavelength conversion based on 4WM, so only a narrow band ofwavelengths near the water absorption are generated, rather than theentire SC spectrum.

Another potential application for the SC generation can be to use theMIRFIL in Optical Coherence Tomography (OCT) systems used in bio-medicalimaging and diagnostics. Over the past two decades, OCT has beenestablished as a diagnostic technique for minimally invasive,high-resolution, cross-sectional imaging in a variety of medical fields.The OCT system comprises a broadband, low-coherence light source, afiber-based Michelson interferometer, a sample scanning and positioningstage, and a detector followed by electronics. OCT is analogous toconventional ultrasonic pulse-echo imaging, except that it does notrequire direct contact with the tissue that is being investigated and itmeasures echo delay and the intensity of the back-reflected infraredlight rather than acoustic waves from internal tissue structures. Thelight source used in OCT helps to determine the instrument properties interms of the spectral bandwidth (axial resolution), center wavelength(penetration depth), power density (data acquisition time), cost andsize. The SC light source that provides broad bandwidth without using amodelocked laser could lead to micron level resolution for OCT systemswithout using an expensive light source.

OCT is usually used for biological systems. However, the SC light sourcecould also be advantageously be used with OCT for sub-surface defectdetection in semiconductors, ceramics, or other solid state materials.As an illustration, OCT could be used to inspect silicon wafers beforethey are processed. This could permit sorting of the wafers (i.e.,charge a premium for better wafers) and avoiding the cost of processingpoor quality wafers. Alternately, OCT could be used to inspectmulti-layered structures. By using SC light beyond 1.1 microns, whichfalls below the bandgap of silicon, the light can penetrate into thechip or wafer. Also, by using longer wavelengths, the scattering loss isreduced. Furthermore, because of the broadband spectrum, the depthresolution of OCT can be at the sub-micron level. Thus, sub-micron toseveral micron sized defects could be inspected using the SC-lightsource based OCT.

Typical OCT systems operate point-by-point, which may be too slow forsome of the wafer or chip inspection applications. The speed is limitedboth because imaging is done point-by-point, as well as because one armof the OCT is moved to achieve the depth resolution. As an alternative,methods used in spectral domain OCT can be used to avoid moving one armof the interferometer, and by using a line scan the point-by-pointscanning could be avoided. As an example, the light from the output ofthe SC source could be stretched onto a line using a cylindrical lens oran appropriate optical lensing system. The resulting line of light couldbe split using a beam splitter to a reference arm with a referencemirror or sample and the sample arm. The sample can be located in thesample arm, and the sample can be moved below the light to scan line byline. The return beams from the reference arm and sample arm can berecombined at the beam splitter, and an imaging lens can then be used toimage it into a spectrometer. In one particular embodiment, thespectrometer could be dispersive optics, such as a grating or a lens,which could take each point of light and spread it into a spectrum to bedetected by a detector array. By processing the multi-spectral data fromeach spatial point, the location of the reflection from the sample canbe detected. Thus, instead of using a movement of the reference arm, theFourier transform of the interference data may be processed to obtainthe height of the reflection.

Yet another application of the SC source is in the so-called “last milesolution” in telecommunications. The last mile solution includes thetechnologies related to fiber-to-the-X (FTTx), where X can exemplary behome, node, neighborhood, curb, or premise. As one example, the SCsource can be an enabling technology for wavelength division multiplexedpassive optical networks (lambda-PONS). In a lambda-PONS based FTTxsystem, each location can receive one or more wavelengths. A challengefor lambda-PONS is the multi-wavelength light source, which may resideat the central office or other telecommunications location. The SCsource can advantageously provide a potentially low-cost solution forthe multi-wavelength light source.

In one particular embodiment, the SC source can be coupled to one ormore modulators and a wavelength division multiplexer to implement theFTTx multi-wavelength light source. As an example, the SC source couldadvantageously emit wavelengths covering the low loss window in opticalfibers, advantageously between 1250 nm and 1750 nm. For this example,the entire SC source could be implemented in fused silica fiber. Then,the output from the SC source can be separated into multiple wavelengthchannels, using, for example, a wavelength division multiplexer. Eachwavelength can then be modulated using a modulator. The modulatedwavelength signals can then be combined and coupled to the output fiberfor propagation over the FTTx system. In addition, in the FTTx systemthe power splitters may be replaced with wavelength divisionmultiplexers. Examples of wavelength division multiplexers includearrayed waveguide gratings, waveguide grating routers, dielectric coatedbeam splitters, and bulk optical gratings.

In another embodiment, the FTTx multi-wavelength light source couldcomprise a SC source coupled to a dispersive pulse stretcher, one ormore high-speed modulators, and a wavelength division multiplexer toseparate the wavelengths. Advantageously, the SC source couldadvantageously emit wavelengths covering the low loss window in opticalfibers, particularly between 1250 nm and 1750 nm. The dispersive pulsestretcher can then broaden the SC pulse, spreading the wavelengths sothe channels occupy different time slots. The one or more high speedmodulators can be used to time sequentially encode the differentchannels, and then the wavelength division multiplexer is used toseparate the wavelength channels in the FTTx system.

Although the present invention has been described in severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfalling within the spirit and scope of the appended claims.

What is claimed is:
 1. An optical system for use in an imagingprocedure, the system comprising: one or more semiconductor diodesconfigured to generate an input beam, wherein at least a portion of theinput beam comprises a wavelength shorter than about 2.5 microns; one ormore optical amplifiers configured to receive at least the portion ofthe input beam and to communicate an intermediate beam to an output endof the one or more optical amplifiers; and one or more optical fibersconfigured to receive at least a portion of the intermediate beam and tocommunicate at least the portion of the intermediate beam to a distalend of the one or more optical fibers to form a first optical beam; anonlinear element configured to receive at least a portion of the firstoptical beam and to broaden a spectrum associated with the at least aportion of the first optical beam to at least about 50 nm through anonlinear effect in the nonlinear element to form an output beam with anoutput beam broadened spectrum; and a subsystem comprising one or morelenses or mirrors configured to receive a received portion of the outputbeam and to deliver a delivered portion of the output beam to a sampleto perform imaging for characterizing the sample, wherein the subsystemcomprises an Optical Coherence Tomography (OCT) apparatus comprising asample arm and a reference arm, wherein the delivered portion of theoutput beam has a temporal duration greater than approximately 30picoseconds, wherein the delivered portion of the output beam has arepetition rate between continuous wave and Megahertz or higher, whereina time averaged output power of the output beam is about 20 mW or more,and wherein a time averaged intensity of the delivered portion of theoutput beam is less than approximately 50 MW/cm².
 2. The system of claim1, wherein at least a portion of the nonlinear element is a light guidecomprising a material selected from the group consisting of ZBLAN,fluoride, tellurite, chalcogenide, semiconductor, high non-linearityglass and photonic crystal glass.
 3. The system of claim 1, wherein atleast a part of the sample is selected from the group consisting ofskin, biological tissue, corneal tissue, semiconductor wafer,semiconductor chip and multi-layered structures.
 4. The system of claim1, wherein the Optical Coherence Tomography apparatus further comprisesa spectral domain OCT system that comprises a Fourier transformprocessing of interference data.
 5. An optical system for use in animaging procedure, the system comprising: a plurality of semiconductordiodes, each of the diodes configured to generate an optical beam; abeam combiner configured to receive at least a portion of the opticalbeams from the plurality of semiconductor diodes and to generate amultiplexed optical beam; and an optical fiber configured to receive atleast a portion of the multiplexed optical beam and to communicate theat least a portion of the multiplexed optical beam to form anintermediate beam having at least one wavelength; a light guideconfigured to receive at least a portion of the intermediate beam and topropagate the at least a portion of the intermediate beam to form anoutput beam; and a subsystem comprising one or more lenses or mirrorsconfigured to receive a received portion of the output beam and todeliver a delivered portion of the output beam to a sample to performimaging for characterizing the sample, wherein the subsystem comprisesan Optical Coherence Tomography (OCT) apparatus comprising a sample armand a reference arm, wherein the delivered portion of the output beamhas a temporal duration greater than approximately 30 picoseconds,wherein the delivered portion of the output beam has a repetition ratefrom continuous wave to Megahertz or higher, wherein a time averagedoutput power of the output beam is about 20 mW or more, and wherein atime averaged intensity of the delivered portion of the output beam isless than approximately 50 MW/cm².
 6. The optical system of claim 5,wherein at least a portion of the light guide comprises a materialselected from the group consisting of ZBLAN, fluoride, tellurite,chalcogenide, semiconductor, high non-linearity glass and photoniccrystal glass.
 7. The optical system of claim 5, wherein at least aportion of the sample is selected from the group consisting of skin,biological tissue, corneal tissue, semiconductor wafer, semiconductorchip and multi-layered structures.
 8. The optical system of claim 5,wherein the Optical Coherence Tomography apparatus further comprises aspectral domain OCT system and a line scan system that comprises acylindrical lens.
 9. An optical system for use in a spectroscopyprocedure, the system comprising: one or more semiconductor diodesconfigured to generate an input beam, wherein at least a portion of theinput beam comprises a wavelength shorter than about 2.5 microns; one ormore optical amplifiers configured to receive at least the portion ofthe input beam and to communicate an intermediate beam to an output endof the one or more optical amplifiers; and one or more optical fibersconfigured to receive at least a portion of the intermediate beam and tocommunicate at least the portion of the intermediate beam to a distalend of the one or more optical fibers to form a first optical beam; anonlinear element configured to receive at least a portion of the firstoptical beam and to broaden a spectrum associated with the at least aportion of the first optical beam to at least about 50 nm through anonlinear effect in the nonlinear element to form an output beam with anoutput beam broadened spectrum; and a subsystem comprising one or morelenses or mirrors configured to receive a received portion of the outputbeam and to deliver a delivered portion of the output beam to a sampleto perform spectroscopy for characterizing the sample based on chemicalcomposition, wherein at least a part of the sample comprises skin ortissue, wherein the delivered portion of the output beam has a temporalduration greater than approximately 30 picoseconds, wherein thedelivered portion of the output beam has a repetition rate betweencontinuous wave and Megahertz or higher, wherein a time averaged outputpower of the output beam is about 20 mW or more, and wherein a timeaveraged intensity of the delivered portion of the output beam is lessthan approximately 50 MW/cm².
 10. The system of claim 9, wherein atleast a portion of the nonlinear element is a light guide comprising amaterial selected from the group consisting of ZBLAN, fluoride,tellurite, chalcogenide, semiconductor, high non-linearity glass andphotonic crystal glass.
 11. The system of claim 9, wherein the chemicalcomposition of the sample comprises protein, amino acids or amidegroups.
 12. The system of claim 9, wherein the spectroscopy forcharacterizing the sample is Fourier Transform Infrared (FTIR),absorption or reflection.
 13. The system of claim 9, wherein thespectroscopy for characterizing the sample comprises spectralfingerprinting, wherein at least the portion of the output beambroadened spectrum is used to illuminate substantially simultaneously anumber of spectral features and wherein the sample is identified by thenumber of spectral features.
 14. The system of claim 9, wherein thespectroscopy for characterizing the sample comprises at least in part afilter based spectroscopy in a point detection apparatus that furthercomprises one or more dispersive optics, slits and detectors, andwherein at least a part of the nonlinear element is a photonic crystalfiber.
 15. An optical system for use in a spectroscopy procedure, thesystem comprising: a plurality of semiconductor diodes, each of thediodes configured to generate an optical beam; a beam combinerconfigured to receive at least a portion of the optical beams from theplurality of semiconductor diodes and to generate a multiplexed opticalbeam; and an optical fiber configured to receive at least a portion ofthe multiplexed optical beam and to communicate the at least a portionof the multiplexed optical beam to form an intermediate beam having atleast one wavelength; a light guide configured to receive at least aportion of the intermediate beam and to propagate the at least a portionof the intermediate beam to form an output beam; and a subsystemcomprising one or more lenses or mirrors configured to receive areceived portion of the output beam and to deliver a delivered portionof the output beam to a sample to perform spectroscopy forcharacterizing the sample based on chemical composition, wherein atleast a part of the sample comprises skin or tissue, wherein thedelivered portion of the output beam has a temporal duration greaterthan approximately 30 picoseconds, wherein the delivered portion of theoutput beam has a repetition rate from continuous wave to Megahertz orhigher, wherein a time averaged output power of the output beam is about20 mW or more, and wherein a time averaged intensity of the deliveredportion of the output beam is less than approximately 50 MW/cm².
 16. Theoptical system of claim 15, wherein at least a portion of the lightguide comprises a material selected from the group consisting of ZBLAN,fluoride, tellurite, chalcogenide, semiconductor, high non-linearityglass and photonic crystal glass.
 17. The optical system of claim 15,wherein the spectroscopy for characterizing the sample is FourierTransform Infrared (FTIR), absorption or reflection.
 18. The opticalsystem of claim 15, wherein the spectroscopy for characterizing thesample comprises spectral fingerprinting, wherein the wavelength of theat least a portion of the output beam is used to produce substantiallysimultaneously a number of spectral features and wherein the sample isidentified by the number of spectral features.
 19. The optical system ofclaim 15, wherein the chemical composition of the sample comprisesprotein, amino acids or amide groups.
 20. The optical system of claim15, wherein the spectroscopy for characterizing the sample comprises atleast in part a filter based spectroscopy in a point detection apparatusthat further comprises one or more dispersive optics, slits anddetectors, and wherein at least a part of the light guide is a photoniccrystal fiber.